Question

the sun went down at last, and the gray day imperceptibly merged into the gray dusk.
source: the mystery of witch face mountain and other stories
in the sentence above, imperceptibly has the same or almost the same meaning as:
irreverently
unnoticeably
invariably
indelicately

Explanation:

"Imperceptibly" means in a way that is not perceptible or noticeable. "Unnoticeably" has a very similar meaning. "Irreverently" means lacking respect. "Invariably" means always. "Indelicately" means in a not delicate or rude way.

Answer:

unnoticeably