The sentence describes a period of dry weather, so we need a noun that fits the idea of a temporary duration, especially of weather.
(a) beauty (noun): Grammatically correct - it fits the noun slot. However, contextually incorrect, as "beauty" refers to aesthetic appeal and has no connection to a period of dryness or weather.
(b) spell (noun): Correct both grammatically and contextually. "Spell" refers to a short period of time, particularly in weather contexts (e.g., "a dry spell"). Perfect fit.
(c) develop (verb): Grammatically incorrect - we need a noun, not a verb. Also contextually irrelevant, as "develop" refers to growth or creation, not duration or weather.
(d) heavy (adjective): Grammatically incorrect - an adjective cannot complete the phrase "a dry (A)". Also, contextually unsuitable, as "heavy" refers to weight or intensity, not duration.
(e) gently (adverb): Grammatically incorrect - an adverb cannot serve as the object of an article ("a"). Also contextually illogical, since "gently" describes manner, not a period of dryness.
Only (b) "spell" is both grammatically correct (noun following an article) and contextually appropriate (a temporary weather condition).
Hence, option (b) is the correct answer.