Final answer:
The additive inverse of the fraction 21/112 is -21/112, which is the number that yields zero when added to the original number.
Step-by-step explanation:
The additive inverse of a number is the number that, when added to the original number, will result in zero. For the fraction 21/112, its additive inverse is the number that when added to 21/112 yields zero. The additive inverse of a number is simply the original number multiplied by -1.
In the case of 21/112, the additive inverse is -21/112:
Thus, the correct answer to what the additive inverse of 21/112 is would be option b. -21/112.