What Type of questions are asked in Viva Voce?

The methodology of questioning varies from examiner to examiner. The normal way of questioning is to start from the experiment you are alloted.
Example:

What experiment are you doing?
Ans: To determine the value of unknown resistance using metre bridge (Don't say just metre bridge or unknown resistance alone; you should tell the complete aim of the experiment.)

What is the principle on which the experiment is based?

Ans: The metre bridge experiment is based on the Wheatstone's Bridge Principle

(You can go on to state the principle; or it will be the next question)

Then the examiner may go on to ask some tricky questions related to Wheatstone's Bridge.

The questions are generally based on what you answer and usually different sections are asked when you do not give a satisfactory answer.

Have good presence of mind and think before you answer.

You can find some sample questions here arranged experimentwise