It is given that a single die is used.
It is required to find the probability of rolling a number less than or equal to 6.
Recall that the probability of an event is given by the formula:

The sample space of a single die is:

Hence, the total number of possible outcomes is 6.
The numbers less than or equal to 6 are:

Hence, the number of favorable outcomes is 6.
Substitute these values into the probability formula:

The answer is D.