Concept:When two past actions are referenced, the earlier action must use the past perfect tense.
Explanation:The sentence mentions two events in the past: meeting John yesterday and not seeing him since Christmas.
The meeting (“met”) is the later action.
The seeing (or not seeing) happened before that meeting — from Christmas until yesterday.
Therefore, the earlier action needs past perfect: “had seen”.
Option A “saw” is simple past; it would imply the seeing occurred at the same time as the meeting.
However, “since Christmas” indicates a duration leading up to the meeting, so simple past is incorrect.
Option B “have seen” is present perfect, used for past actions with present relevance.
Since the time reference is “yesterday” (a specific past time), present perfect is not appropriate.
Option D “have been seeing” is present perfect continuous, also unsuitable for a completed past time frame.
Only past perfect correctly shows that the seeing occurred before the meeting.
Thus, the most suitable word is “had seen”.
Answer:had seen (Option C)