The problem asks how the circumference of a circle can be calculated using $\pi$. The options are: (a) By multiplying the diameter by $\pi$. (b) By adding the diameter to $\pi$. (c) By subtracting the diameter from $\pi$.

1. Problem Description

The problem asks how the circumference of a circle can be calculated using $\pi$. The options are:

(a) By multiplying the diameter by $\pi$.

(b) By adding the diameter to $\pi$.

(c) By subtracting the diameter from $\pi$.

2. Solution Steps

The formula for the circumference of a circle is given by

$C = \pi d$

where $C$ is the circumference and $d$ is the diameter.

The circumference of a circle is also given by

$C = 2\pi r$

where $r$ is the radius of the circle.

The diameter is twice the radius, so $d = 2r$.

From the formula $C = \pi d$, we can see that the circumference is calculated by multiplying the diameter by $\pi$.

3. Final Answer

By multiplying the diameter by pi.