Correct : c

The original statement is: "When the teacher is in the room, all students stand silently." In logic this is T → S where T = teacher in room and S = all students stand silently.
Option A — Always true
This is the contrapositive — if any student is NOT standing silently then the teacher cannot be in the room. The contrapositive is logically equivalent to the original statement and is always true.
Option B — Always true
The original says students "stand silently" — silence is part of this. So when the teacher is present, students are certainly silent (along with standing). This is a weaker claim than the original and must be true.
Option C — Not necessarily true
This is the converse of the original — concluding the teacher is present whenever all students are standing. The converse of a true statement is not necessarily true. Students might all stand for other reasons — a school assembly, another authority figure entering, or simply out of habit — without the teacher being present. The original gives no information about what happens when students stand independently. C is not necessarily true.
Option D — Always true
Standing is a component of "standing silently." If all students stand silently when the teacher enters, then they are certainly standing. This is always true.
Correct answer: C ✓