Which term of the A.P. 24, 21, 18, ............ is the first negative term?

Answer: Option C

a_n = a + (n - 1) × d where d = -3, Let a_n = 0
⇒ 0 = 24 + (n - 1) × -3
⇒ 0 = 24 - 3n + 3
⇒ 3n = 27
⇒ n = 9
⇒ 10^th term will be negative (-ve)