Transverse elastic waves can propagate:

As gases are not rigid, or their Modulus of rigidity is zero, transverse waves cannot propagate through them.

But in metals which are solids, transverse wave can propagate. 

Transverse elastic waves, unlike longitudinal waves, involve the oscillation of particles perpendicular to the direction of wave propagation. These waves can only propagate in solids that have sufficient rigidity to support the perpendicular motion of particles.

• In metals, which are solid and have strong interatomic bonds, transverse waves can easily propagate because of their ability to support shear forces.
• In gases, however, transverse waves cannot propagate because gases lack the necessary molecular structure and bonding to transmit shear forces. The particles in gases are too far apart, and they can only support longitudinal waves (where particles move parallel to the wave direction).

Thus, the correct answer is C: in a metal but not in a gas.