In the UPSC/IAS Civil Services Main Examination (written), candidates are allowed to choose one optional subject.
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Do you know the preferred choice of the most aspirants for optional subject? Can you guess? Well, the most preferred choice as Optional Subject in UPSC Civil Services Main Exam is:

Geography.
Geography has become the most preferred subject among the optional subjects opted by the candidates, followed by History, Public Administration and Sociology.
Among the candidates who qualify to write UPSC Mains (written exam) every year, around 3000 candidates prefer Geography as the optional subject.
Geography is chosen by more than 3000 candidates who write UPSC Mains Number of candidates who wrote Mains with Geography as optional UPSC Prelims 2019 Test Series - Enroll Now and Get 40% OFF!
• 3391 candidates (in UPSC Mains 2015)

• 3515 (in UPSC Mains 2014)
history History is chosen by more than 1500 candidates who write UPSC Mains The number of candidates who had written UPSC Mains with History as optional
• 1821 candidates (in UPSC Mains 2015)
• 1560 candidates (in UPSC Mains 2014)

Public Administration
Above 1500 candidates who appeared for UPSC Mains chose Public Administration as the optional subject. On yearly basis, the number of UPSC aspirants who appeared in Mains with Public Administration as optional subject are:

• 1795 candidates (in UPSC Mains 2015)
In 2015, 1795 candidates in UPSC Mains

• 2852 candidates (in UPSC Mains 2014)
In 2014, 2852 candidates in UPSC Mains

Sociology 
Sociology is chosen by around 1500 candidates who write UPSC Mains The number of candidates who had written UPSC Mains with Sociology as optional: 
• 1479 candidates (in UPSC Mains 2015)
• 1819 candidates (in UPSC Mains 2014)
Are candidates choosing the subject of their graduation as their optional subject?
Analysing the previous year trends, it can be seen that candidates prefer a Humanities subject as an optional irrespective of their background – engineering, medicine, science, or humanities. In 2015, the influx of aspirants were 55.2% from Engineering, followed by 25.3%, 13.7% and 5.8% from Humanities, Medical Science and Sciences respectively. However, 84% of optional subjects chosen by the candidates were related to Humanities (including literature of languages), followed by 7.2%, 6.1% and 2.7% from fields like Science, Medical Science and Engineering respectively.

It is quite evident that most of the aspirants have switched their streams say from Medical Science, Engineering and other Sciences to Humanities. Can you identify the specific reason behind this switching to Humanities?

It’s actually a herd mentality.
Earlier most of the coaching institutes had only a few faculties for taking optional subjects. Their focussed subjects were Public Administration, History, Geography, Sociology etc. Whichever academic background candidates belonged to, these ‘civil service gurus’ brainwashed candidates to pick one of these optional subjects. They spread the notion that so optional subjects are high scoring while others are not. Are there easy or high-scoring optional subjects?
There is nothing absolute as which of the subjects are easy or difficult.

Some candidate may find History interesting while another may find it mundane.
The UPSC has a mechanism of scaling or normalising various optional subjects to ensure parity. Thus, choosing popular optional subjects does not guarantee qualification.
How should you choose your UPSC Mains optional subject?
We recommend the following process:
Go through the previous year UPSC CSE Mains question papers (at least 3 years) of the optional subjects you like. Also, go through the UPSC syllabus of optional subjects. Which subject do you feel comfortable now? Do you have a passion for that subject?
Will you be able to find study materials (online or offline for the same) and practise model question papers?

Choose the subject of your interest as optional subject irrespective of it being included in your graduation or not. Best Wishes!