The stereochemical relationship of H_a and H_b in the following molecule is

Concept:

Diastereotopic Ligands:

• Two atoms or groups in a molecule are said to be diastereotopic if the sequential replacement of these two atoms or groups by a new atom or ligand results in the formation of a pair of diastereoisomers.
• When the 2 atoms or groups in a molecule are adjacent to a chiral center or present in a chiral environment, these 2 ligands can be identified as diastereotopic ligands.

Explanation:

• The two H atoms H_a and H_b in the molecule are replaced by D in the molecule in the subsequential steps and it results in the formation of two different isomers.
• When Ha in the molecule is replaced by the D atom, it gives the isomer A. While the replacement of Hb in the molecule results in the isomer B.
• Now, the mirror image of isomer A generates a new isomer C, which is an enantiomer of isomer A. Thus, the newly formed isomers A and B, do not have any mirror image relationship.​

Conclusion:

Hence, the stereochemical relationship of H_a and H_b in the following molecule is diastereotopic.