Solve the Vertical Addition: 90 + 9 in Column Format

To solve this addition problem, follow these steps:

• Step 1: Align the numbers vertically. We have the numbers $90$ and $9$.
• Step 2: Write them down with equal alignment such that their units digits are aligned:

$\begin{aligned} &90 \\ + &~9\\ \end{aligned}$

• Step 3: Begin by adding the rightmost column (units column): $0 + 9 = 9$. Write $9$ in the units place below the line.
• Step 4: Move to the left column (tens column). Add: $9 + 0 = 9$.
• Step 5: Write $9$ in the tens place below the line.

This gives us the complete sum:

$\begin{aligned} &90 \\ + &9 \\ \underline{~~~~} \\ &99 \\ \end{aligned}$

Therefore, the sum of $90$ and $9$ is $\textbf{99}$.

Referring to the multiple-choice answers provided, the correct choice is \textbf{choice 4: 99}.