Verified answer

1250 + d(n-1) = 50

50 is the 4th term so n is 4.

1250 + d(3) = 50

d = -400

a_k is the 2nd term, so n is 2. 1250 + -400(2-1) = 850.

I don't think that any of your answer choices are right.

a = 1250

4th term = 1250+(4-1)d = 50

d = -400

Sequence is

1250, 850, 450, 50

a_k = 850