To determine the number of terms in the arithmetic sequence where the first term \( u_1 = 2 \), the common difference \( d = 4 \) (since \(6 - 2 = 4\)), and the last term \( u_n = 90 \), we can use the formula for the \( n \)-th term of an arithmetic sequence:

\[
u_n = u_1 + (n - 1) \times d
\]

Step-by-Step Calculation:

1. Plug in the values:

\[
90 = 2 + (n - 1) \times 4
\]

2. Simplify the equation:

\[
90 - 2 = (n - 1) \times 4
\]

\[
88 = (n - 1) \times 4
\]

3. Solve for \( n \):

\[
n - 1 = \frac{88}{4} = 22
\]

\[
n = 22 + 1 = 23
\]


The number of terms in the sequence is 23.

The correct answer is D. 23