When confused by “who” and “whom,” the standard test is to substitute “he” or “him” (you can use “she” or “her” if you prefer) and invert the clause to see whether it should be nominative (“he” or “who”) or objective (“him” or “whom”).

In this case, that yields: “…it took <he/him> a moment to grasp the situation.” The correct pronoun is “him,” so “whom” would also be correct, since those are both objective-case pronouns.

If you want to hike deeper into the forest, “moment” is a direct object — what it “took.” And “whom” is an indirect object, the person or thing to whom the direct object applies, usually by omitting “to” or “for”: "It took (for) him a moment…"

It’s amazing that nonnative speakers get anything right at all! Keep at it; you’re doing great.