The missing term in the sequence 1, 9, 25, 49, 81 is 121. This is because the sequence is a list of squares of the first six natural numbers, 1, 2, 3, 4, 5, and 6. The square of a number is the product of that number and itself. So, the square of 1 is 1, the square of 2 is 4, the square of 3 is 9, and so on. Therefore, the missing term in the sequence is 121, which is the square of 11.
Option A, 100, is not the missing term because it is not a square of a natural number. Option B, 112, is not the missing term because it is not a square of a natural number. Option C, 121, is the missing term because it is the square of 11. Option D, 144, is not the missing term because it is not a square of a natural number.