The correct subject-verb agreement from the answer choices below is A)is...has.
'A cake or brownies is the easiest to serve. My mom has agreed to bake this weekend.'
That sounds better than
'A cake or brownies are the easiest to serve. My mom have agreed to bake this weekend.'
'A cake or brownies is the easiest to serve. My mom have agreed to bake this weekend.'
'A cake or brownies are the easiest to serve. My mom has agreed to bake this weekend.'
Those don't make sense when you read them, hence why A) is the correct choice.