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.
thype the correct form of define.
baskin-Robbins_______a new flavor every month.
I hope this helps. Thanks for asking.