B. Baking a cake is easy, but baking muffins is hard.
In writing/grammar, parallel structure is when the same parts of speech (or even just the same pattern) are used in a sentence such as on both sides of a coordinating conjunction or in a list. For instance, let’s look at a list that is parallel. This list is parallel because the list items are all gerunds:
I like running, sleeping, and eating.
If one of the list items happened to be a preposition with its object, such as “to sleep” in the following list, it could not be considered parallel:
I like running, to sleep, and eating.
Thus, the option that has the same general pattern on both sides of the coordinating conjunction is the following:
Baking a cake is easy, but baking muffins is hard.
