This question tests the correct preposition of time used with the present perfect sense "have been ill" when a specific starting point in time is mentioned.
- Option A (From): "From" can indicate a starting point but is not the idiomatic choice with the perfect tense describing a state that continues to the present; "from" is more typically paired with "to" for a range of time.
- Option B (Since): "Since" is used with the present perfect tense to mark a specific point in time from which an action or state has continued up to now. "Monday last" (last Monday) is a specific point in time, so "since" is the correct choice.
- Option C (On): "On" is used for a specific day or date, but not with the perfect tense sense of an ongoing state measured from that day to now.
- Option D (Of): "Of" is not a time preposition used with "ill" in this context.
The correct answer is B) Since, so option 2 is the answer.