What is the expression for the circumference of a circle with diameter d?

To determine the expression for the circumference of a circle given its diameter (d), we refer to the fundamental relationship between the diameter, radius, and circumference of a circle. The circumference of a circle is defined as (C = \pi \times d), where (d) is the diameter.

In this case, since we've established that the relationship is direct and straightforward, the circumference can be calculated simply by multiplying the diameter by (\pi). This clearly shows that the correct expression for the circumference of a circle when utilizing the diameter (d) is indeed expressed as (C = \pi d).

This understanding stems from the formula itself and is pivotal in solving problems related to circles in mathematics. The other expressions presented in the choices do not accurately represent the circumference based on this fundamental geometric principle.