The correct answer: "My calculations lead me to conclude that the answer is 5040'. I therefore don't understand where I may have gone wrong."
5,040' feet is, in principle, a 'less safe' answer than the 5,105' in B, as you're erring 65' on the 'dangerous' side for your planning. But, as binzer has observed, and as you will find in the real world, there are hundreds of feet in a pencil width and rounding assumptions. Just avoid any operation in which 65' either way make a real difference.
Qantas put a Jumbo into a golf course - or was it a rice paddy? I forget - but I reckon that 65' either way wasn't the cause.