If a proton is moved a certain distance directly opposite an external electric field, what can we say about the change in the electric potential it experiences?

Potential energy increases.

Explanation:

We know that the direction of electric field is from positive to negative charge. As the proton has the positive charge so if it moves in the direction opposite of electric field, it means that the positive charge will move towards the positive region. As the positive charge is attracted towards negative and repelled by the positive charge. So the work done will be negative in bringing the positive charge towards the positive region of the field and potential energy increases in the direction opposite to electric field. As the potential energy decreases in the direction of electric field.

the answer is b, have negligible mass