Final answer:
The digit that makes the sentence true is 4.
Step-by-step explanation:
To determine which digit makes the sentence true, we need to compare the numbers 668.65 and 668.6. The digit that goes in the blank should make the statement true. In this case, 668.65 is greater than 668.6, so the blank should contain a digit that is greater than 5. Therefore, the answer is D. 4.