Final answer:
The correct adjective to use is 'bounciest' as it is the superlative form of 'bouncy' suitable for the comparison implied in the sentence.
Step-by-step explanation:
To correct the adjective error in the sentence, we should choose an adjective form that can be used for comparison. The sentence involves potential comparison by implying that Black Panther's rubber band ball might be unequalled in its bouncing quality compared to other creations in town. Out of the given options (a) Bounciest, (b) Bouncy, (c) Bouncer, and (d) Bounceful, the correct choice is (a) Bounciest. This is because 'bounciest' is the superlative form of 'bouncy,' which indicates the highest degree of the adjective when comparing more than two things or groups. Therefore, the correct sentence would be: If Black Panther keeps adding to his rubber band ball, he will have the most bounciest creation in town.