Final answer:
A constant is a number that doesn't change, such as the speed of light or Planck's constant. Constants are used in scientific equations to represent fixed values, unlike variables which can vary.
Step-by-step explanation:
A constant is: a) A number that doesn't change. This is the correct answer because in mathematics and sciences, a constant refers to a fixed value that does not change. For instance, the speed of light (c = 2.99×108 m/s), Planck's constant (h), and Avogadro's number (NA) are all constants used in scientific equations and calculations. Similarly, in physics, when discussing motion, it's common to assume a constant acceleration, which simplifies calculations and is a valid approximation in many real-world scenarios. Moreover, a rate constant in chemistry refers to a particular value characteristic of a specific reaction under a set of conditions, and it does not change as the reaction progresses.
In contrast, a variable can take on different values, and a mathematical operation refers to procedures such as addition, subtraction, multiplication, and division. Letters in the alphabet are used as symbols to represent variables or constants, but they are not constants themselves.