A ring of DNA has 9000 base pairs. Enzyme A cuts it at 1500 and 5000…

Step 1. (a) A ring cut twice gives 2 pieces: $5000 - 1500 = 3500$ and $9000 - 3500 = 5500$.

Step 2. (b) For B: $7500 - 3000 = 4500$ and $9000 - 4500 = 4500$. The two pieces are equal, so they run as one band.

Step 3. (c) Together the cuts are at 1500, 3000, 5000 and 7500. The pieces are $1500$, $2000$, $2500$, and $9000 - 7500 + 1500 = 3000$. The largest is 3000.