To find the number of terms in the arithmetic sequence where the first term $U_1=2$, the common difference $d=4$, and the last term is $U_n=90$, use the formula for the $n$-th term of an arithmetic sequence:

$$U_n=U_1+(n-1)\times d$$

Step-by-Step Solution:

1. Substitute the given values into the formula:

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

2. Solve for $n$:

$$90-2=(n-1)\times 4$$
$$88=(n-1)\times 4$$
$$n-1=\frac{88}{4}=22$$
$$n=22+1=23$$


The number of terms in the sequence is 23.

The correct answer is D. 23.