We have the formula S= r * theta , where S is arc length , r is radius , and theta is central angle in radians
So AB= 4 * 45* π/180 = π units
varies directly means 
inserting y = -18 and x = 3 ⇒
⇒ -6 = k
next, insert k = -6 into the varies directly equation and solve for y. y = -6x
Answer: C
The answer is A) 3.
I hope this helps.