Addition Problem: Finding the Sum of 3 Units and 9 Units

To solve this problem, we will follow these steps:

• Step 1: Identify the numbers given in the problem.
• Step 2: Use addition to find the sum of these numbers.
• Step 3: Verify that the sum corresponds to one of the answer choices.

Now, let's work through each step:
Step 1: The problem gives us 3 units and 9 units.
Step 2: We'll use the addition operation.
$3 + 9 = 12$
Step 3: Check the given choices to ensure our answer, 12, matches one of them.

Therefore, the solution to the problem is $12$.