In the passage, the SECOND and fifth sentences are GIVEN. This can give us more information about the FIRST and the sixth sentences.

The first sentence always introduces a topic and is independent in its meaning.

Here, the passage talks about habits and habituation. As the second sentence uses the pronoun 'they', it MUST be referring to 'habits' in the first sentence. Only sentence R is independent and talks about what habits are.

So, R must be the first sentence.

The fifth sentence introduces 'habituation'. Only P talks about 'habituation' and gives more information about it. 

This means P is the last or sixth sentence.

The two remaining sentences must have a logical SEQUENCE to them. Here, Q talks about giving an example of brushing teeth while S actually asks the question that Q mentions. This means, QS is the logical order.

So, Q is the third sentence and S is the fourth sentence in the sequence.

The correct sequence then becomes: RQSP or R2QS5P.

Therefore, the correct answer is option 5.