Question

A dipole is placed in a uniform electric field with its axis parallel to the field. It experiences
(a) only a net force
(b) only a torque
(c) both a net force and torque
(d) neither a net force nor a torque

Answer 1

 A dipole is placed in a uniform electric field with it axis parallel to the field , It experience___ Option (d) is correct :- neither a net force nor a torque.

Refer to the attachment , Reason for All options

 Reason (a):- Only a Net force

See the attachment,
Since, when the +ve charges moves across to the electric field. and -ve charges moves in the opposite direction of electric field. Then the forces are equal & opposite hence the Net Force is zero. Hence angle b/w axis of dipole and field is zero , because dipole is placed towards field direction.
So it is the wrong option.

Reason (b) :- Only a Torque :-  

As we know Torque is vector product of the given force and it's perpendicular distance. 
∴ Torque τ = f s sin q. 
Here q   is the angle b/w electric field and it's axis of dipole. 
∵ they are on angle to the applied force, Then sin 0 = 0 
∴ Torque is also zero. τ = 0

Hence this is the wrong option.

Reason (c) :- As we discussed here, so option c is also wrong.

Reason (d):- Neither a net force nor a Torque , This option is right.
Refer to the option a and b. 

Hope it Helps.

Answer 2

Answer:

(d) neither a net force nor a torque.