The answer is an appositive phrase. An appositive is an extra piece of detail or information describing in this case "lemon cake" and which the sentence still makes sense without. It still makes sense to say "<span>For my birthday, we baked a lemon cake." The "special treat" adds additional information about the "lemon cake."</span>
You did not give the full text so I cannot be sure of the answer, but A makes the most sense with what I could read.
Okay so 6×3 is equivalent to 15+3 because both equations give you the answer if 18.
