We know that
l = a + (n - 1)d
∴ 21 = -5 + (n - 1) × d
⇒ (n - 1)d = 21 + 5
⇒ (n - 1)d = 26
⇒ d = n−126.
The formula for sum of A.P. is given by,
Sn=2n[2a+(n−1)d]∴Sn=2n[2×(−5)+(n−1)×n−126]⇒200=2n[−10+(n−1)×n−126]⇒200=2n[−10+26]⇒2n×16=200⇒8n=200⇒n=8200⇒n=25.
Hence, Option 4 is the correct option.