Final answer:
The enol form of 5-bromouracil pairs with guanine due to the capability to form the necessary hydrogen bonds, therefore guanine is the correct nucleotide that will base-pair with the enol form of 5-bromouracil.
Step-by-step explanation:
5-Bromouracil (5BU) is a nucleoside analogue to thymine nucleoside. When 5BU is in its enol form, it is capable of hydrogen bonding with the nucleotide guanine (G). This is because the enol form has a structural layout that can form the necessary hydrogen bonds with guanine. In the context of base pairing in DNA, guanine always pairs with a complementing pyrimidine; and when 5BU (mimicking thymine) is in its enol form, it pairs with guanine instead of adenine, which thymine normally pairs with. This can lead to a nucleotide pair conversion from an AT base pair to a GC base pair after replication. Therefore, the correct answer is (b) Guanine.