Hi there, I'll help you out :)
The expression that is equivalent to 5^3 • 5^x is A. If you solve the expression given in the question, you get 5^3 + x, which is exactly the same as the first choice.
- B gives you 15625
- C gives you 125x^3
- D gives you 15625^x
- E gives you 148,877^x
- F can't really be solved
Because none of the other answer choices match up, that leaves A as your only answer.
Hope this helps!