Do you understand why "defines" is correct?
Even though "Baskin-Robbins" appears to be plural (with the -s on the end), it's singular because it's the name of one company. Therefore, the verb needs to have the third-person-singular ending (-s) when in present tense.
http://grammar.ccc.commnet.edu/grammar/verbs.htm#tense
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thype the correct form of define.
baskin-Robbins_______a new flavor every month.
defines
I hope this helps. Thanks for asking.