When an electric field is applied across a semiconductor
 

a. electrons move from lower energy level to higher energy level in the conduction band
b. electrons move from higher energy level to lower energy level in the conduction band
c. holes in the valence band move from higher energy level to lower energy level
d. holes in the valence band move from lower energy level to higher energy level

1. (a, c)

2. (a, d)

3. (b, d)

4. (c, d)

(a, c) Hint: The motion of the electrons depends on the force on the electrons.
Step 1: Find the motion of the electrons.
When an electric field is applied across a semiconductor, the electrons in the conduction band get accelerated and acquire energy. They move from lower energy level to high energy level.
Step 2: Find the motion of the holes.
While the holes in the valence band move from higher energy level to lower energy level, where they will be having more energy.