What is the difference between em waves and mechanical waves?

Electromagnetic waves differ from mechanical waves in that they do not require a medium to propagate. This means that electromagnetic waves can travel not only through air and solid materials, but also through the vacuum of space.

Explanation:

Since electromagnetic waves do not require a medium to pass through, they are faster in matter that has fewer particles. ... ~ Sound cannot travel in a vacuum because mechanical waves, such as sound waves, require a medium to propagate through.

Option C

Explanation:

In his experiment, Milgram wanted to test the extent of obedience a person can go as per the instructions of the authority.  

In this he concluded the experiment saying that several factor when changed in the second round of experiment led to the dropping of obedience level and these factors are as follows –  

a)Conducting the experiment at some ordinary place instead of Yale university campus

b)When the teachers were asked to force the hand of learner to touch the shock plate and they were placed in the same room

c)Two participants in the same room – If one participants showed disobedience then the other participant’s disobedience level will fall automatically

d)When the authority giving instructions is not nearby or is absent

Hence, option C is correct