I like that -- bossy verb

I wouldn't consider this a "command subjunctive"... but "command subjunctive" is just a term made up to help you remember that one of the structures that requires subjunctive is a command ("The prof demanded that we be in our seats by 8:00, for example"). The important takeaway is just that you need subjunctive 'be' in the ice cream sentence, as opposed to the indicative 'is.'

(However, subjunctive in "whether"-type constructions is actually often considered archaic nowadays, and I'd be very surprised if the GMAT forced you to make a choice as to whether to use it in these types of constructions.)