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Answer: B - Books are used to collect secondary data.

Secondary data is the data already available. They have already been collected by someone and may be published or unpublished. Books, journals, reports, letters etc. are examples of secondary data.

Other methods mentioned in the choices are used for primary data collection.

Projective techniques are used in psychology to use projection to infer underlying motives or intentions. This technique is most importantly followed in motivational research or in attitude surveys.

Questionnaires are generally used in enquiries. The set of questions are sent and requested to be returned with an answer.

Observation method is mainly used in behavioral studies.