If Caiyan ate pizza tomorrow, then she would become fat.

This is a great test of tense.

a. If Caiyan ate pizza tomorrow, then she would become fat.
- "If X then Y" correct use of the hypothetical subjunctive.

b. If Caiyan eat pizza tomorrow, then she will become fat.
- Wrong conjugation of the verb 'to eat.' Future tense is used, and therefore the verb should be 'ate'

c. She would become fat if Caiyan eats pizza today.
- The pronoun is used in an ambiguous manner before Caiyan. Who is 'She'?

d. If Caiyan had eaten pizza tomorrow, she would have become fat.
- Wrong use of the past perfect tense, had eat tomorrow doesn't work gramatically as the 'had' indicates actions started and completed in the past.

e. If Caiyan ate pizza yesterday, she would be fat.
- Incorrect construction of "If ... Then..."