I shall stay here __________ Sunday.
Tripura Public Service Commission - 2019,
Answer: C
The preposition is till: I shall stay here till Sunday.
Till (or until) marks the end point of a continuing action: the staying goes on up to Sunday and then stops.
By Sunday means "not later than Sunday" and suits a single action to be completed (finish it by Sunday), not a continuing stay. For needs a length of time (for a week). Since marks a starting point in the past and goes with the perfect tense, not with shall stay.
A quick test: till fits when the action continues up to a point; by fits when it is completed at or before that point.
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