18) B

Total number of parts = 7 + 1 = 8
The maximum age would be when the rock contained 8 parts of ²³⁵U.

Here are the half lives:

Half – life ²³⁵U ²⁰⁷Pb
080
144
226
317

So it was after 3 half-lives that the ratio became 1 : 7

Half-life = 7.1 x 10⁸ years
So maximum age of rock = 3 x 7.1 x 10⁸ = 21.3 x 10⁸ = 2.13 x 10⁹ years