Final answer:
Option C, which is 10, is the constant because it is the only number not associated with any variable. The other options include variables and their values can change, hence they are not constants.
Step-by-step explanation:
The question asks to identify which of the given options is a constant. In mathematics, a constant is a fixed value that does not change. 10 is the only number among the options that is not associated with any variable and hence is a constant. The other options, 4xy, -7b, and 18bc^3, are not constants because they include variables (x, y, b, c) whose values can change.
The correct answer is C. 10.