which is the best optional subject for engineering students??

here are the list of the subjects given below which you can take in the mains examinatin.

Agriculture,

Animal Husbandry and Veterinary Science,

Botany, 

Chemistry, 

Civil Engineering,

Commerce and Accountancy, 

Economics, 

Electrical Engineering, 

Geography, 

Geology,

History, 

Law, 

Management, 

Mathematics, 

Mechanical Engineering,

Medical Science, 

Philosophy,

Physics, 

Political Science and International Relations, 

Psychology, 

Public Administration, 

Sociology,

Statistics, Zoology.
but i think you should take mathematics and physics 
because engineering students are sharp in maths and physics.