The correct answer is $\boxed{\text{B. }2^n}$.
A truth table is a mathematical table used in logic to show the functional values of logical expressions on each of their functional arguments, that is, for each combination of values taken by their logical variables. In particular, truth tables can be used to show whether a propositional expression is true for all legitimate input values, that is, logically valid.
The number of input words in a truth table is equal to the number of possible combinations of values that can be assigned to the input variables. For a truth table with $n$ input variables, there are $2^n$ possible combinations of values.
Option A is incorrect because $10$ is not a power of $2$.
Option C is incorrect because $4$ is not a power of $2$.
Option D is incorrect because $8$ is not a power of $2$.
Option E is incorrect because it is not one of the possible answers.