Which principal part of the verb best completes this sentence? i confessed that i had all the leftovers.
a. eat
b. eaten
c. ate
d. eating

Sometimes, reading things out loud can help a lot.  "I had eat all the leftovers."  Does that sound right?  How about "I had eating all the leftovers," or "I had ate all the leftovers"?  What about "I had eaten all the leftovers"?

If this doesn't work for you, think of it like this: you must always use the PAST PARTICIPLE after a form of the word "have."  For example, "I had given you the present."  Which of your options is most similar?  Most likely, the answer is "eaten": "I confessed that I had eaten all the leftovers."

B. Eaten