Four streets enter a traffic circle at points a b c and d name six arcs formed by the streets.

Arcs are just sections of the circle
so lets say you had a pizza

and you put some points on the crust, the legnth of the crust from point to point is that arc

example
if you put poins A and B, then arc AB is the crust between points A and B

arcs can include other points as well
 and arc AB=arc BA

so
I wil name easy arcs first
arc AB
arc BC
arc CD
ard DA

now ones that include others
arc AC
arc BD

so the arcs you might want to list are
 
arc AB
arc BC
arc CD
ard DA
arc AC
arc DB