ব্যাখ্যা
The word "better" is versatile in English — it can function as an adjective, adverb, or verb depending on context.
Analyzing each option:
Option (A): "hoping for better weather"
— "better" modifies the noun "weather"
— Noun modifier = Adjective ✗
Option (B): "Sound travels better in water"
— "better" modifies the verb "travels"
— Verb modifier = ADVERB ✓
— It answers the question: "How does sound travel?" → "better"
Option (C): "decide which one is better"
— "better" is a predicate adjective (subject complement after "is")
— Adjective ✗
Option (D): "to better his health"
— "better" functions as a VERB meaning "to improve"
— It is an infinitive verb here ✗
Summary of "better" as different parts of speech:
— Adjective: better weather, a better idea
— Adverb: runs better, sleeps better, travels better
— Verb: to better oneself, to better one's position
— Noun: for the better, the better of the two
Source: Longman Dictionary of Contemporary English; Oxford Advanced Learner's Dictionary.