The formula used to find the circumference \( C \) of a circle when the radius \( r \) is given is:
\[ C = 2\pi r \]
Among your options, the correct formula resembles \( C = x2r \) where \( x \) can be understood to represent \( \pi \).
So if we consider \( x \) as \( \pi \), then:
\[ C = \pi \times 2 \times r \]
Therefore, the choice that best represents the formula is:
C = xr (if we consider \( x \) to mean \( 2\pi \)).
To clarify, the common expressions for circumference are typically noted as \( C = 2\pi r \).