The sentence discusses how a rise in humidity may have caused or initiated the precipitation. So, the blank requires a verb that conveys the idea of triggering or initiating an event.
(a) strength (noun): Grammatically incorrect - a noun cannot function as the main verb in this sentence. Contextually inappropriate - "strength" refers to power or physical force, not to a cause or catalyst for an event.
(b) musical (adjective): Grammatically incorrect - an adjective cannot serve as a verb. Contextually irrelevant - even as a descriptor, "musical" doesn't relate to weather events or causation.
(c) happily (adverb): Grammatically incorrect - an adverb cannot act as the main verb in the sentence. Contextually meaningless - "happily" describes manner or mood, not causation.
(d) sparked (verb - past tense): Grammatically correct - fits the passive voice construction ("may have been sparked"). Contextually accurate - "sparked" means initiated or caused, which is perfect in the context of a chain reaction starting with volcanic eruptions.
(e) paper (noun): Grammatically incorrect - a noun in this position cannot act as the main verb. Contextually unacceptable - "paper" has no relation to humidity, weather, or geological triggers.
Only (d) "sparked" is both grammatically and contextually appropriate to fill the blank meaningfully.
Hence, option (d) is the correct answer.