The correct answer is $\boxed{\text{A}}$.
The series is $2, 3, 5, 8, 13, \boxed{21}, 34$. The next term in the series is found by adding the previous two terms and subtracting 1. For example, $8 + 13 = 21$ and $21 - 1 = 20$.
Option $\text{B}$ is incorrect because $25$ is not the next term in the series.
Option $\text{C}$ is incorrect because $29$ is not the next term in the series.
Option $\text{D}$ is incorrect because $34$ is already in the series.
