Verified Answer :

B30 

if you go down 1,000 then that means -10
if you go up 3,000 than that means +30

It would be D: -30
Because decrease = a negative integer

A 10-degree decrease for every 1000 meters in elevation would be: -10 1 = -10
Since this is 3000 meters, it's -10
3 = -30