= 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 are
$$
\boxed{H_a,\ H_b,\ H_c,\ H_d\ \text{all axial}.}
$$
The <chemical shifts> alone do not determine this assignment. In particular, three unequal splittings of a methine are not a geminal-coupling pattern.
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