1st number of each term is forming a series of prime number in increasing order. ∴Next number = 13 Last number of each term is also forming a series of prime number is decreasing order. ∴ Next number = 7 And middle letter (number) = Sum of 1st & 3rd number. ⇒Middle term (letter) =(13+7=20) ⇒ T ∴ 13 T 7 ⇒ Option 1 is correct.