Step 1: List what each device physically senses:
Instead of matching one item at a time, it helps to first write down the underlying physical quantity that each device in GROUP X actually senses. A fusible plug loop does not sense heat, smoke or gas directly at all; it only senses the loss of pneumatic pressure that occurs once its low melting point element has melted. A thermal and smoke detector senses two things together, a temperature rise and airborne combustion particles. A gas detector senses the concentration of flammable vapour in the air around it.
Step 2: Compare this list against GROUP Y one statement at a time:
Statement (I) talks about detecting a rise in temperature together with visible and invisible particles of combustion, which is exactly the two-part sensing job of a thermal and smoke detector, so (I) goes with Q. Statement (II) talks about a mechanism that works on a drop in pressure to activate shutdown, which is exactly how a fusible plug loop operates once the alloy melts, so (II) goes with P. Statement (III) talks about detecting flammables, which is the job of the gas detection system, so (III) goes with R.
Step 3: Cross check against the given options:
The matching obtained is P-II, Q-I, R-III. Scanning the four options, this combination is exactly option (B), while the remaining options each swap at least one pairing incorrectly, for example option (A) wrongly assigns temperature and smoke sensing to P instead of a pressure based mechanism.
Step 4: State the final match:
The correct grouping is confirmed as P-II, Q-I, R-III.
Final Answer:
\[ \boxed{P-II;\ Q-I;\ R-III\ \text{(Option B)}} \]