Final answer:
The correct synonym for 'manifest' in the context of 'Hymn to the Nile' is 'reveal,' which signifies making something evident or known, similar to how the flood's effects are made apparent.
Step-by-step explanation:
A synonym for manifest as it is used in the lines from Hymn to the Nile is reveal. The phrase 'Verdant the spirit at your coming, O Flood' implies that the coming of the flood is made known or shown to others, which aligns with the definition of reveal. Conceal, create, and destroy do not accurately capture the sense of making something clear or evident to the senses or mind, as manifest does in this context. Therefore, the correct option is 1) reveal.