Answer: B
Correct answer: Fluorescent
The correct spelling is fluorescent - fluor-, not flour-. The transposition is almost universal, because the English word flour is far commoner and the ear supplies its order. But the two are unrelated: this word comes from fluorspar, the mineral in which the phenomenon was first observed, which is itself named from the Latin fluere, to flow, because it was used as a flux in smelting. The same fluor- appears in fluoride, fluorine, fluorescence and fluorocarbon, and the same fluere in fluid, fluent, influence and affluent. The ending is -escent, which marks a beginning or a process, as in adolescent, convalescent, luminescent and effervescent. The noun is fluorescence, with the same shift from -ent to -ence.