Final Answer:
-9 < 9 matches the description, "negative 9 is less than 9" (option a).
Step-by-step explanation:
The correct math sentence that matches the description ""negative 9 is less than 9"" is -9 < 9. In this expression, the symbol ""<"" indicates ""less than."" Negative 9 is indeed less than 9 (option a).
To break it down, consider the position of -9 and 9 on the number line. -9 is to the left of 9, indicating that it is smaller or less than 9. The ""less than"" symbol aligns with the direction of the number line, reinforcing the fact that -9 is positioned before 9.
In mathematical terms, the statement -9 < 9 can be read as ""negative 9 is less than 9."" The numerical value of -9 is smaller than 9, validating the correctness of the expression. Therefore, option (a) -9 < 9 is the correct answer, matching the given description.