Triangle ABC is similar to triangle PQR, as shown below:
Which ratio is equal to c:r?
b:q
c:a
r:b
q:a

we know that
If triangle ABC is similar to triangle PQR
then
the corresponding sides are proportional
so
[tex]\frac{AC}{PR}= \frac{AB}{PQ}= \frac{BC}{QR}[/tex]
substitute the values
[tex]\frac{b}{q}= \frac{c}{r}= \frac{a}{p}[/tex]
[tex]\frac{c}{r}=\frac{b}{q}[/tex]
[tex]\frac{c}{r}=\frac{a}{p}[/tex]
therefore
the answer is the option
[tex]b:q[/tex]