Restriction enzyme digestion
Examine the effects of Enzyme A on its own.  

Enzyme A performs two cuts that result in 2 fragments that are 5000bp long.
We know that the first cut is made at the 100th bp.  Therefore the second cut is made at the 5100th bp (A2).

You can stop there and determine that the answer is C.  C is the only answer that states A2 = 5100.