Correct choice is C:

If and whether are used to show conditions. If is usually used for multiple conditions or "If .. not" IDIOM.

As usually is used to compare clauses but I believe in this case C is the right choice since the sentence denotes the action of eating again and again... you will know. Also note "as .. often" IDIOM

Also, the Idiom is " If <present> (then) <future>"
If you eat <present>..., you often find <present> - which is incorrect. The correct usage would be "If you eat...., you will ..."

D is wrong because If....then construct is not followed.

If + present sentence+ then+ will + base verb (ofcourse then is optional in the sentence)