Explanation:

To solve this problem, let's consider each sentence fragment and their potential sequence:

- S1 and S6 are fixed as the beginning and ending sentences.

- P, Q, R, S need to be ordered correctly to form a coherent passage.

Let's analyze the options:

1. Option 1: a

- Sequence: P, Q, R, S

- Each fragment logically follows the previous one, flowing seamlessly in thought.

2. Option 2: b

- Sequence: Q, P, S, R

- This might disrupt the logical flow, as some parts need more context to be clear.

3. Option 3: c

- Sequence: R, S, P, Q

- This sequence may jumble the natural progression of ideas.

4. Option 4: d

- Sequence: S, R, Q, P

- Thought development isn't as intuitive, leading to confusion.

- When analyzing, Option 1 (a) fits best as it ensures the middle sentences transition smoothly.

?? Correct Answer: Option 1 (a)