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."
Answer: B. Eaten
        
             
        
        
        
Native Americans, also known as American Indians, Indigenous Americans and other terms, are the indigenous peoples of the United States, except Hawaii and territories of the United States. More than 570 federally recognized tribes live within the US, about half of which are associated with Indian reservations.
        
             
        
        
        
C. Combine it with a sentence fragment
        
                    
             
        
        
        
Hello!
The correct answer is C. "People who support animal testing are crazy."
This is an example of an ad hominem fallacy because the thesis states that animal testing is valuable, which insinuates that the person making the claim believes in animal testing. By claiming that "people who support animal testing are crazy" is making a statement about the person himself rather than about the argument. The sentence is stating that the person who supports animal testing is crazy rather than explaining a counter-argument about the topic. 
I hope this helps you! Have a lovely day!
- Mal