Select the appropriate preposition to fill in the blank in the following…

English prepositions of time follow patterns based on how precise the time reference is: "at" introduces an exact point in time, such as a specific clock time or a moment like midnight or noon; "in" introduces a longer or less precise period, such as a month, a year, a season, or a part of the day; "from" marks the starting point of a span of time, usually paired with "to" or "until" for the endpoint; and "about"/"around" is placed before a time expression only when the sentence is explicitly hedging or estimating (words like "I think", "roughly", "nearly", "probably"), to show the time given is approximate rather than exact.

The sentence "Our school breaks up exactly ____ 4 p.m." explicitly states that the closing time is exact ("exactly"), ruling out any approximate reading. An exact clock-time reference marked as "exactly" calls for the exact-point-in-time preposition, so the blank must take "at".

• "from" would fit only if the sentence gave a starting point paired with an end point, e.g. "from 9 a.m. to 4 p.m."; there is no such pair here.

• "in" would fit a broader period such as a month, year, season, or part of the day ("in the evening"), not a specific clock time.

• "about" signals an approximate time, and this directly contradicts the word "exactly" already in the sentence ("exactly about 4 p.m." would be self-contradictory); so "about" does not fit here.

Therefore, the correct preposition is "at", giving: "Our school breaks up exactly at 4 p.m."