Well the boom is in equlibrium so you know that the total torque about any point on the boom is equal to zero.
Therefore I would suggest you pick the point labeled B, because then you have eliminated one unknown (the torque from the cable).
You know how much torque the block makes, and how much torque from the center of mass of the beam makes.
So there is just one unknown left, the torque from the pivot.
Hope that helps.