firstly, the negative sign indictaes that this is a decay. Secondly, it is decaying by 31% (or 31 per 100) per annum, or 0.31, however, I dont think any of these answers are right. certainly C and D represent growth as they are 131% and 169% (1.31 and 1.69 respectively), B represents a decrease of 4% or 0.04, and A represents a decrease of 87% or 0.87, which as you can see does not correspond to to a decrease of -0.31or -31%, are you sure you have supplied all the information in the question(such as a starting point). If you have then I would refer to teacher/tutor for further examination.