One approach is to choose an arbitrary value for the number that Maurice FIRST entered into his calculator BEFORE he accidentally divided by 0.03 (instead of 0.0003)
Let's say that 0.03 is that FIRST value.
So, Maurice INTENDED to divide 0.03 by 0.0003. IF he had performed the correct operation, he would have calculated 0.03/0.0003 = 100.

INSTEAD, Maurice accidentally divided 0.03 by 0.03. So, Maurice's incorrect calculation = 0.03/0.03 = 1

So, what must we do to the incorrect answer (1) in order to get the correct answer (100)?
Let's check the answer choices...

A. Multiply the incorrect product by 100. If we multiply 1 by 100, we get 100. PERFECT!
B. Divide the incorrect product by 100. If we divide 1 by 100, we get 0.01. NO GOOD
C. Multiply the incorrect product by 0.01. If we multiply 1 by 0.01, we get 0.01. NO GOOD
D. Divide the incorrect product by 0.01. If we divide 1 by 0.01, we get 100. PERFECT!

Answer: A & D

Cheers,
Brent