​ Which expression represents θ in terms of x?

When you use the sin function, you 'undo' it when you use the sin inverse (also called the arcsin) function     so   if you start with theta

    then you use the sine function ,  sin (theta)     then    arcsin ( sin theta) = theta

Your equation:   sin(theta) = x/10      now    arcsin both sides

       arcsin(sin (theta))  = arcsin  (x/10)

                 theta = arcsin (x/10)