Total balls = 5 + 8 + 7 = 20. Neither blue nor pink means only yellow balls are favourable. Number of yellow balls = 7.
$$P(\text{neither blue nor pink}) = \frac{7}{20}$$
Answer: C $\dfrac{7}{20}$
"Neither blue nor pink" means the ball must be yellow. Count yellow balls (7) over total balls (20). Eliminate the other two colours — don't add them to the favourable count.