Protons and neutrons are made from combinations of the two most common quarks, the u quark and the d quark. the u quark's charge is +2/3e, while the d quark carries −1/3e.
part a
how could three of these quarks combine to make a proton?

a. uuu
b. uud
c. udd
d. ddd
part b
how could three of these quarks combine to make a neutron?

a. uuu
b. uud
c. udd
d. ddd

Part A  B.  Part B C.

Explanation:

A) The elementary charge e (without sign) is equal to the charge of one electron (with negative sign) or to the charge of one proton (with positive sign), so the proton must have a total charge of +e.

if u = +2/3 e and d= -1/3 e, we need a combination which sum gives +3/3 e.

Combination A adds to 6/3e, so it is not possible. C adds to zero, and D gives a negative result.

The only remaining choice is udd:

uud ⇒ +2/3 e + 2/3 e -1/3 e = +3/3 e = +e

So, the statement B is true.

B) As the neutron has no net charge, we need to find a combination which sum adds to zero.

So, A and D are not possible, as they are combinations of the same type of quarks, so the sum is either positive or negative, but not zero.

uud gives +e (we chose it to make a proton in part A), so the only remaining choice is udd:

udd⇒ +2/3 e -1/3 e - 1/3 e = 0

So the statement C is true.