Complete the Two-Digit Composite Number: Finding _9

To solve this problem, we'll proceed with the following steps:

• Step 1: Identify possible candidates for the digit filling $\square$.
• Step 2: Form numbers $29, 39, 59,$ and $79$ using the candidates $2, 3, 5,$ and $7$ respectively.
• Step 3: Test each number for composite status by checking for divisibility aside from 1 and itself.

Let's examine the numbers:

Step 1 and Step 2: Candidates give us the numbers $29, 39, 59,$ and $79$.

Step 3: Check each number:
- $29$ is only divisible by 1 and 29 (prime).
- $39$ is divisible by 1, 3, 13, and 39; hence, it is composite.
- $59$ is only divisible by 1 and 59 (prime).
- $79$ is only divisible by 1 and 79 (prime).

Therefore, the number $39$, formed by filling $\square$ with 3, is composite.

Thus, the correct number to fill in the blank is $3$.