The correct answer is (C) A precursor; a harbinger, forerunner, or herald; something that gives warning; one who announces the approach of another. Also, anything that foreshadows a future event; omen; sign. Finally, one formerly sent in advance of an army, royal train, etc., to secure lodgings and other accommodations.

Harbinger comes from Middle English herbergere (to provide shelter), Old French herbergere, herberge (lodging), herbergier (to lodge), Old High German heriberga (hari [army] + berga [shelter]).

Examples of use:

• The sound of a bird beyond the widow was a harbinger of spring.
• The sudden wind was the harbinger of a fast approaching thunder storm.