Best approach in such queries is to look for each answers, we know we cannot substitute the value of a in 8^a from 2^a = b.

So , we go to answers straight.

look for options- A says b^3 which is 2^a ^3 = 8^a.

So in this case we dont look further and go to the right answer which option A.

When questions and answers both are in variable in PS, subtituting with numbers such as 2/3, may not be a quick idea.

Bunuel wrote:

Tough and Tricky questions: Exponents.

If 2^a = b then 8^a must equal to

A. b^3

B. b^2

C. 3b

D. b^(2a)

E. b^(3a)

Kudos for a correct solution.