 Solution

The diagnostic feature is the large vicinal coupling characteristic of an axial–axial relationship in a chair. The td patterns for $H_a$ and $H_c$ contain two large couplings to neighbouring axial hydrogens, so both are axial. The ddd patterns for $H_b$ and $H_d$ also include a large ring vicinal coupling, with the smaller values reflecting the other ring hydrogen and the side chain. The assignments areThe chemical shifts alone do not determine this assignment. In particular, three unequal splittings of a methine are not a geminal-coupling pattern.