To find the probability, you have to divide the amount of black horses in the pasture by the total amount of horses.
Black horses in the pasture = 5
Total amount of horses = 39
5 ÷ 39 = 0.128
0.128 = 12.8%
12.8% rounded to the nearest whole percent = 13%
Answer = 13%(1st Choice)
Hope this helped :)