(A) "hearing aid" → A hearing aid is a medical device that helps people with hearing impairment understand speech.
(B) "listening to classical music" → "Listen to" is the correct phrase when actively paying attention to music.
(C) "dredging the banks" → Dredging means clearing sediment/debris from water bodies, necessary before monsoon season.
(D) "building a sandcastle" → "Building" is the appropriate verb for constructing a sandcastle.
Hence, Option 1: (A) - (III), (B) - (IV), (C) - (I), (D) - (II) → Each verb correctly completes its sentence both grammatically and contextually: hearing aid (medical device), listening to music (active engagement), dredging river banks (maintenance work), and building sandcastle (construction activity) → correct