I look into all the answers
A is giving you the total of 30 kilograms
B is telling you that it gains the average of 170 which that one looks right
c is giving you the 170 average and the kilograms per year
D is stating the gains a total of 170 kilo to first 3 years
So in my opinion, the correct answer is
B. The slope is 170. This means that the horse gains on average 170 kilograms per year for the first years of its life.