Final answer:
The coordinating conjunction in the sentence is 'but'.
Step-by-step explanation:
The coordinating conjunction in the sentence is but. Coordinating conjunctions are used to join two independent clauses or sentences. In this particular sentence, the coordinating conjunction 'but' is used to connect the two independent clauses 'I wanted to buy a cute little pumpkin' and 'Batman insisted on getting the biggest one.'
Learn more about Coordinating conjunctions