It grieved me _______________ my friend’s serious illness.

The phrase begins with ' It grieved me ' — here, It is a dummy subject, and grieved is the main verb. After such expressions, English usually expects an infinitive clause (to + verb), not a gerund phrase.

So, after “It grieved me,” the correct complement is to hear (infinitive), not hearing (gerund).

The correct structure is It + verb (emotion) + object + infinitive clause

In option B, 'regarding' is used as a preposition. However, the prepositions 'of' and 'about' are the correct prepositions used with the verb 'hear'.

Hence, option D is the correct choice.