The correct answer is $\boxed{\text{D}}$.
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 $2^n$, where $n$ is the
number of inputs. In this case, there are 6 inputs, so there are $2^6 = 64$ input words.Option A is incorrect because it is the number of input words for a 4-input OR gate.
Option B is incorrect because it is the number of input words for a 3-input OR gate.
Option C is incorrect because it is the number of input words for a 2-input OR gate.
Option E is incorrect because it is not a valid option.