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.

To check their medical records to see if they have had chicken pox before, if they haven’t, monitor their temperature, drink plenty of fluids, and stay away from small children and those who haven’t been exposed to chicken pox during the incubation period.