The correct options are
A the magnitude of buoyant force acting on the block is
50N. B the spring potential energy is
12.5J.There are three forces when the block is in equilibrium.
1. weight of the block downward.
2. buoyancy force due to the liquid upward
3. force due to the spring upward
Weight of block: mbg=Vbρbg=Vb×2000×10
Buoyancy force upward: Fbuoyancy=Vbρlg=Vb×1000×10
Upward force due to spring: Fspring=kx=100x
Equating the upward force with the downward force,
Vb×1000×10+100x=Vb×2000×10
∴100x=Vb×10000=102000×10000
∴x=12m
Potential Energy stored in the spring: PE=12kx2=12×100×(12)2=12.5Nm=12.5J