You are correct, the answer is B. 7 ... however let's figure out why this is.
Unstable isotopes such as carbon 14 undergo spontaneous radioactive decay. Depending on the isotope, it will undergo a certain type of decay. Carbon 14 is known to undergo beta radiation and emit what is known as a beta particle. This beta particle is in the form of an electron, written:
The top number here, 0 is indicative of our mass (an electron has no mass). The bottom number -1 is indicative of our charge (electrons have a negative charge).
C --> ? + e
So we need to balance our beta emission equation here. All mass and charges must balance. So we have +6 charge on the left side of the equation. What number must we put in the ? for that to balance on the right? The answer is in fact 7 since we have a -1 in the form of a beta particle. For the mass to balance we must have a 14 there as well.
So which element has a 7 atomic number? The answer is nitrogen. The following equation is balanced:
14 14 0
C --> N + e
6 7 -1
Did you know that this is the chemical process that occurs for scientists to determine how old an organic substance is? This is called carbon dating!