117
121
123
129
Answer is Right!
Answer is Wrong!
The answer is $\boxed{\text{D}}$.
The given series is an arithmetic progression with a common difference of $6$. This means that each term is $6$ more than the term before it. The first term is $3$, so the 21st term is equal to $3 + (21 – 1) \times 6 = 129$.
Option A is incorrect because $117$ is not a term in the given series.
Option B is incorrect because $121$ is not a term in the given series.
Option C is incorrect because $123$ is not a term in the given series.