Step 1: Understand the problem
The problem is asking us to use four 9s and basic mathematical operations to get as close to 100 as possible.

Step 2: Explore different mathematical operations
To solve this, we can use addition, subtraction, multiplication, and division、We need to think creatively about how to combine these operations with the four 9s.

Step 3: Try different combinations
Let's start with simple addition: 9 + 9 + 9 + 9 = 36, which is less than 100.

Step 4: Experiment with different operations
Using division and addition: 99 + 9 / 9 = 99 + 1 = 100.

Step 5: Validate the solution
The expression 99 + 9 / 9 equals 100 because 9 divided by 9 equals 1, and 99 + 1 = 100.

The final answer is: $\boxed{99 + 9 / 9}$