CONCEPT: The question tests the basics of Geometry(Circle).
Given ∏R2 =R
=> R = 1/∏ =7/22
(Now we know 1/22 as a percentage is 4.16 and hence 7/22 will be 4.16*7 = 29.12%)
Closest option as a decimal is (b) as (a) is anyhow smaller than 29.12 and the rest of the options are very far from 29.12%)