Solitary savings bank received an initial sepout

Can someone me do math because i am having a breakdown rn because i don’t get it

Apinhole camera is made out of a light-tight box with a piece of film attached to one side and pinhole on the opposite side. the optimum diameter d(in millimeters) of the pinhole can be modeled by d=1.9[ (5.5 x 10^-4) l) ] ^1/2, where l is the length of the camera box (in millimeters). what is the optimum diameter for a pinhole camera if the camera box has a length of 10 centimeters

Which equation represents a direct linear variationa. y=x2b. y=2/5xc. y=1/xd. y=x+3