What is the right word to use here: Buying and selling antiques can be a very ____________ hobby if you know how to find a good bargain.

The correct choice is "profitable." This word describes a hobby that can generate financial gain. In the context of buying and selling antiques, the phrase suggests that if someone is knowledgeable about finding good deals, they can make a profit from their activities. The adjective "profitable" directly relates to the idea of earning money, making it the best fit for the sentence.

The focus is on the potential positive financial outcome of the hobby, which aligns well with the context of being skilled at finding bargains in the antiques market. The other choices, such as "profitless" and "unprofitable," suggest a lack of financial benefit, which contradicts the intended meaning of the sentence. "Profitably" is an adverb and does not fit grammatically with the structure of the sentence, which requires an adjective to modify the noun "hobby." Overall, "profitable" is the most suitable word to convey the positive aspect of the hobby in this context.