Final answer:
The right word to complete the sentence with regard to the sound of thunder is 'sporadic', which suggests it's occurring at irregular intervals.
Step-by-step explanation:
The correct word to fill in the blank in the sentence 'Someone banged on the door across the hall loud enough for Gavin to hear over the TV and the ___ thunder.' is sporadic. This term implies that the thunder is occurring at irregular intervals, which aligns with the context given in the question. The other options (wistfully, reprieve, and burden) do not logically fit with the description of thunder sounds.