Explanation:

Statement 1: They are not mentioned in the Constitution. However, the Rules of Business provide for their establishment. They are of two types—standing and ad hoc. The former are of a permanent nature while the latter are of a temporary nature. They are set up by the Prime Minister according to the exigencies of the time and requirements of the situation. Hence, their number, nomenclature, and composition varies from time to time.

S2: They not only sort out issues and formulate proposals for the consideration of the Cabinet, but also take decisions. However, the Cabinet can review their decisions. They are an organisational device to reduce the enormous workload of the Cabinet.

S3: Their membership varies from three to eight. They usually include only Cabinet Ministers. However, the non-cabinet Ministers are not debarred from their  membership. They not only include the Ministers in charge of subjects covered by them but also include other senior Ministers.

S4: They are mostly headed by the Prime Minister. Sometimes other Cabinet Ministers, particularly the Home Minister or the Finance Minister, also acts as their Chairman. But, in case the Prime Minister is a member of a committee, he invariably presides over it.