The correct answer is A. constant.
A linear cost function is a function of the form $C(x) = ax + b$, where $a$ is the fixed cost and $b$ is the variable cost. The fixed cost is the cost that is incurred regardless of the level of output, while the variable cost is the cost that varies with the level of output.
In a linear cost function, the fixed cost is considered as a constant because it does not change with the level of output. The variable cost, on the other hand, does change with the level of output.
Option B is incorrect because the variable cost is the cost that varies with the level of output. Option C is incorrect because the exponent is not a term in a linear cost function. Option D is incorrect because the base is not a term in a linear cost function.