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