Select the option that can be used as a one-word substitute for the given group of words.
Social organisation where males are heads of families

Let's look at the definitions of the terms provided:
Patriarchy: A system of society or government in which men hold the power and women are largely excluded from it; a social organization where males are heads of families.

Matriarchy: A system of society or government ruled by a woman or women.

Polygamy: The practice or custom of having more than one wife or husband at the same time.

Hierarchy: A system in which members of an organization or society are ranked according to relative status or authority.

Hence, Patriarchy is the correct substitute.