Do not use a comma before and to separate two verbs that have the same subject
Do you mean "Do not use a comma to separate two verbs that have the same subject"? Where did you get this rule from? It seems a bit strict to me.
Remember that GMAT will never test you on comma usage alone - it's far too controversial and subjective an area of grammar. You're right that we usually wouldn't use a comma between two parallel elements. For example
For dinner I ate bread and cheese.
However, if the sentence is long it might be clear to use a comma. For example
For dinner I ate bread that had been lying in the cupboard for so long that it had green mold growing on it and smelt disgusting, and cheese.
However, in answer B the parallel structure has three elements, so we'd use a comma to separate them in the form "x, y, and z". GMAT follows the convention called the Oxford comma, always using a comma before and when a list has three or more items.
