P : when a chemical substance Q : the food poisoning occurred R : in the food preparations S : was mistaken for salt and used The Proper sequence should be:

The correct answer is A. PSRQ.

The sentence is about food poisoning that occurred when a chemical substance was mistaken for salt and used in food preparations. The correct sequence of the sentence is:

1. A chemical substance was mistaken for salt.
2. The chemical substance was used in food preparations.
3. Food poisoning occurred.

The other options are incorrect because they do not follow the correct sequence of events. Option B, QPSR, is incorrect because it places the event of food poisoning before the event of the chemical substance being mistaken for salt. Option C, RQPS, is incorrect because it places the event of the chemical substance being used in food preparations before the event of the chemical substance being mistaken for salt. Option D, SRQP, is incorrect because it places the event of food poisoning after the event of the chemical substance being used in food preparations.