Unit Digit Problem: Identifying Numbers Ending in 7

Which number has a unit digit of 7?

To solve this problem, let's examine the options provided and identify the number that ends in 7:

• Option 1: $71$ - The units digit here is 1.
• Option 2: $79$ - The units digit here is 9.
• Option 3: $17$ - The units digit here is 7.
• Option 4: $16$ - The units digit here is 6.

Looking at the units digit of each option, we can see that option 3 is the only number with a units digit of 7.

Therefore, the number that has a unit digit of 7 is $\boxed{17}$.