Choose the correct word(s) to fill in the blanks. __________ the eve…

Concept: English relies on a large set of fixed prepositional phrases (collocations) — such as "on the eve of" an event, "at" a transit hub, or "en route to" a destination — where the preposition is fixed by convention with the surrounding words rather than derived from one universal rule. These phrases must be recognised as whole units; substituting a different preposition, even one that sounds plausible in isolation, breaks the fixed expression.

Applying it: Filling each blank means matching it to the fixed phrase it belongs to:

1. "____ the eve ____ the Prime Minister's visit" refers to the period immediately preceding a major event — this fixed phrase takes the preposition used for occasions and events, not the one used for an exact clock-time or a deadline.

2. The second blank in that same phrase stays constant across every option ("of"), linking "eve" to the event it precedes.

3. "tight security ____ the airport" places security presence at a transit hub — this takes the preposition used for being present at a specific point or hub, not the one used for enclosure within a larger space.

4. "en route ____ the venue" — "en route" is a fixed phrase that is always followed by the preposition marking a stated destination.

Combining the four words needed for these fixed phrases gives exactly the sequence in the keyed option: "On; of; at; to."

Comparing the choices:

• "At; of; in; along" — opens with the point-in-time preposition (as in "at noon"), which does not attach to "eve" in this fixed phrase, and closes with "along," which describes moving alongside something rather than arriving at a stated destination.

• "By; of; at; in" — opens with the deadline-marking preposition (as in "submit by Friday"), a sense that does not fit a period immediately preceding an event, and ends with the enclosure preposition for "the venue" rather than the one "en route" requires.

• "In; of; in; to" — opens with the enclosure/broader-time preposition (as in "in the morning"), which likewise does not attach to "eve" here, and repeats that same enclosure preposition for "the airport," where the transit-hub phrase requires a different one.

Only the sequence that keeps every blank inside its own fixed phrase reads naturally end to end, matching the keyed option "On; of; at; to."