The key idea being tested here is the physical meaning of the "shear center", so check what happens for each option using that meaning.
- Only torsion: torsion appears only when the shear load line is offset from the shear center. Since \(P\) is applied exactly at the shear center, this offset is zero, so there is no twisting moment at all. This option is wrong.
- Only bending: whenever a transverse load is applied to a cantilever, it always creates an internal bending moment and shear force along the span, whether or not the section is symmetric. Combined with the fact that the twisting moment is zero (because the load passes through the shear center), the beam bends without any twist. This matches the physics exactly.
- Both torsion and bending: this would be the outcome if \(P\) acted away from the shear center, for example at the centroid of an unsymmetric section. Since the problem places \(P\) at the shear center itself, torsion drops out, so this option is wrong.
- Neither torsion nor bending: a transverse end load on a cantilever always bends it, this cannot be true.
The shear center is defined precisely so that a shear force through it produces zero torque, so the unsymmetric shape of the section does not matter for this particular load position, only bending results.
Let's summarize:
- Shear center = point where a transverse load causes bending with no twist, by definition.
- An unsymmetric section still avoids torsion if the load is placed exactly at that point.
So the beam undergoes only bending, option (B).