Explanation:

- Let's break down the pattern using the terms and numbers:

- U144 : S121: The letters decrease by 2 (U to S), and the numbers are square numbers with 144 = 12² and 121 = 11².

- N100 : L81: Again, the letters decrease by 2 (N to L), and the numbers are square numbers with 100 = 10² and 81 = 9².

- G64 : ?: Following the pattern, decrease the letter G by 2 to get E.

- The number 64 is 8², following the pattern, decrease by one square to get 49, which is 7².

- Therefore, the correct option should be: E49.