What form of birth control os 100 percent effective

The correct answer is:  "abstinence".

  Note:  "Abstinence" is, in fact, the only "100%" effective form of birth control".
If one "abstaining" (i.e., resisting, not participating in, refraining) from "having sex" [particularly, penile-vaginal sex; as well as various other risky activity, then one "cannot get pregnant" or "cannot get someone pregnant."

{Note:  since if one does not "have sex", one [a female] cannot
   "get pregnant"; and one [a male] cannot "get someone pregnant" .

Note that this answer is not a "silly" or "non-sense answer".  
Note that "abstinence" is not only considered "100% effective" — but also considered specifically a "[100% effective] form of birth control " — in MANY health education/sex education classes, manuals, instructor manuals, agencies such as Planned Parenthood.

1. a lack of knowledge