In a class of 30 students, 18 play football and 12 play basketball. If 5 play both sports, how many play neither sport?

Let $N(F)$ be the number of students who play football, $N(B)$ be the number of students who play basketball, and $N(F \cap B)$ be the number of students who play both sports.
The number of students who play at least one sport is given by $N(F \cup B) = N(F) + N(B) - N(F \cap B)$.

$$N(F \cup B) = 18 + 12 - 5$$ $$N(F \cup B) = 30 - 5$$ $$N(F \cup B) = 25$$

So, 25 students play at least one sport.

The total number of students in the class is 30.
The number of students who play neither sport is the total number of students minus the number of students who play at least one sport.

$$\text{Neither sport} = \text{Total students} - N(F \cup B)$$ $$\text{Neither sport} = 30 - 25$$ $$\text{Neither sport} = 5$$

The number of students who play neither sport is 5. This corresponds to option A.

The final answer is $\text{A. 5}$.