Choose the word (or words) that would best replace the words highlighted in…

In a word/phrase substitution question, the replacement must match the underlined phrase in both sense and grammatical role, not just topic. A word that names a one-time EVENT (a ceremony, a specific day) is not interchangeable with a word that names a RECURRING date tied to that event, even when both relate to the same topic — here, marriage.

The sentence counts ‘the 25th year’ of something happening in their marriage, which only makes sense if that something returns every year. As written, ‘marriage day’ literally names the one specific day the couple got married — a single day cannot have a ‘25th’ occurrence. The word for the yearly return of that date is ‘anniversary’, so ‘marriage anniversary’ — the noun phrase for the yearly recurrence of the wedding date — is the correct replacement: ‘They will be celebrating the 25th year of their marriage anniversary,’ i.e., their 25th wedding anniversary.

A substitution question tests whether the chosen word preserves the MEANING of the underlined phrase, not just whether a mechanical swap reads most smoothly — so surface fluency of the resulting sentence should not be the deciding factor.

Checking each distractor by its own meaning confirms only one term names a yearly-recurring date:

• ‘marriage’ names the ongoing married relationship/institution itself, not a date; although swapping it in reads smoothly, it drops the day/date-recurrence sense of the original phrase, so it cannot be ‘the 25th’ of anything.

• ‘marriage function’ names the wedding ceremony/reception — a single social event held once, at the time of the wedding, not something that returns every year.

• ‘wedding’ is another name for that same one-time marriage ceremony, again not a date that recurs annually.

Because only ‘marriage anniversary’ expresses a date that returns every year, it alone matches the ‘25th year’ framing in the sentence.