Explanation:

- P: Water pollution occurs when

- Q: harmful substances often chemicals or microorganisms contaminate

- R: a stream, river, lake, ocean, aquifer, or other body of water,

- S: degrading water quality

- T: and rendering it toxic to humans or the environment.

Now, let's analyze the options to find the correct sequence:

- Option 1: PQRST

This sequence logically follows the progression of water pollution definition and its impact.

- Option 2: QPRST

Starts with 'harmful substances,' missing the introductory part on water pollution.

- Option 3: STPQR

Begins with effects ('degrading water quality'), making it awkward and incomplete.

- Option 4: SRPQT

Similar to option 3, starts with an effect instead of introducing the topic.

- Option 5: RPQST

Starts with location details, not the main topic of water pollution.

Thus, the correct sequence is: PQRST