The function f(a) represents the cost of a pizza.
The function g(b) represents the delivery charge on the cost.
PART A.
Having said that, a represents the cost of a pizza, b represents the cost of a pizza and sales tax, f(a) represents the cost of a pizza and sales tax, g(b) represents the delivery fee. At last, g(f(a)) represents the cost of the pizza, sales tax, and delivery fee.
PART B.
Let's find the composition of functions
Hence, the answer to part B is the last option.