4, 9, 25, 49, 121, ?

Key idea: each term is the square of a consecutive prime number.
2^2 = 4
3^2 = 9
5^2 = 25
7^2 = 49
11^2 = 121
The next prime after 11 is 13, so the next term is 13^2 = 169.
Answer: 169