Fill in the blank with the appropriate preposition:

The last time we discussed the matter was ____ the 14th of June last year.

on

The correct preposition to use in the blank is on (OPTION 1)

The preposition on is used to indicate a specific point or date in time. In this case, the last time we discussed the matter was on the 14th of June last year, which means that we discussed the matter on that specific date.

The other options are incorrect.

• At can be used to indicate something that is being discussed or talked about, or it can be used to indicate movement or placement near something.
• In is used to indicate location or position, but it is not the most appropriate preposition to use in this case.
• For is used to indicate duration of time, but it is not the most appropriate preposition to use in this case.