Final answer:
The missing punctuation marks in the given sentence are a capital letter, a comma, and an apostrophe.
Step-by-step explanation:
The missing punctuation marks in the sentence are a capital letter at the beginning of the sentence, a comma after 'cupcakes' and an apostrophe after 'Alicia's'.
The corrected sentence should be: 'We are making cupcakes because it's Alicia's birthday.'
These punctuation marks help clarify the sentence structure and indicate the ownership of the birthday.
Learn more about Punctuation in English sentences