Barb's Aunt Gave Her $100 for Her Birthday with the Condition

Barb's aunt gave her $100 for her birthday with the condition that Barb buy herself something.In deciding how to spend the money,Barb narrows her options down to four choices: Option A,Option B,Option C,and Option D.Each option costs $100.Finally she decides on Option B.The opportunity cost of this decision is

A) the value to Barb of the option she would have chosen had Option B not been available.
B) the value to Barb of Options A,C and D combined.
C) the average of the values to Barb of Options A,C,and D.
D) $100.