Given a math problem with context and a question and 5 answer choices, the task is to provide the correct answer choice based on the problem. You must choose one of the given answer choices by letter: a, b, c, d, or e; anything else is invalid.

Problem: if a drawing has an outer circle of diameter 30 and an inner circle of diameter 24, then what fraction of outer circle ' s surface is not covered by the inner circle ?
Options: ['a. 3 / 6', 'b. 8 / 24', 'c. 14 / 16', 'd. 9 / 25', 'e. 2 / 3']
d