While solving a problem, tran calculates an answer that has coulomb-volts as the units.

which quantity did tran calculate?

a. electric potential energy

b. electric potential difference

c. electric field

d. electric potential

Apparently, the question is looking for A. electric potential energy;

but I don't think that's quite right. Electric potential difference is expressed in Joules / Coulomb which is the work to move a charge between 2 points

Example: If the electric field between, say, between 2 capacitor plates is

E = 100 Newtons / Coulomb then the work done in moving a unit of charge from the negative plate to the positive plate separted by 1 cm is

V = E * d = 100 Newtons / Coulomb * .01 meters = 1 Newton-meter / Coulomb

= 1 Joule / Coulomb    which is the electric potential or potential difference

(The definition of electric potential between points is "the work moving a unit positive test charge from one point to the other")

Now in our above example where V = 1 Joule / Coulomb

if we move 10 Coulombs from the negative plate to the positive plate

W = V Q  = 1 Joule / Coulomb * 10 Coulombs = 10 Joules

where work done has the correct units of Joules.

Your textbook should help clarify this.

a. are