Explanation:
The correct option to fill in the blank is option 'D', forward.

Explanation:
When we say that something has been brought forward, it means that it has been rescheduled or moved to an earlier date or time. In this context, the meeting has been rescheduled to Monday instead of its original date or time due to the seriousness of the situation.

Example:
Let's consider a scenario where the original date of the meeting was set for Wednesday. However, due to the seriousness of the situation, it was decided to reschedule the meeting to an earlier date, which is Monday. Therefore, the meeting has been brought forward to Monday.

Other Options:
The other options given are 'on', 'out', and 'down'. Let's analyze why these options are incorrect:

- Option 'A' - on: This option is incorrect because it does not convey the meaning of rescheduling or moving the meeting to an earlier date or time. The preposition 'on' is used to indicate a specific day or date, but it does not imply any change in the original schedule.

- Option 'B' - out: This option is incorrect because it does not convey the meaning of rescheduling or moving the meeting to an earlier date or time. The preposition 'out' does not fit in the context of rescheduling a meeting.

- Option 'C' - down: This option is incorrect because it does not convey the meaning of rescheduling or moving the meeting to an earlier date or time. The preposition 'down' is not used to indicate a change in the schedule of a meeting.

Therefore, the correct option to fill in the blank is 'D', forward, as it accurately conveys the meaning of rescheduling or moving the meeting to an earlier date or time.