Final answer:
The underlined part of the expression '7y + 3x - 9' is '-9', which is a number that stands alone without a variable, making it a constant.
Step-by-step explanation:
The underlined part of the expression 7y + 3x - 9 is -9, which is a number without a variable attached to it. Therefore, the correct answer is C) Constant. A constant is a value that does not change. In an algebraic expression, a constant is a term that is not multiplied by a variable and remains the same. In contrast, a coefficient is a numerical factor of a term with a variable, a variable is a letter representing a number, and a term is either a single number or a variable, or numbers and variables multiplied together.