Does the verb after 'a set of + plural noun' agree with 'set' or 'noun'? For example:
'Law is a set of rules that govern/governs society.'
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In a main clause where "a set of rules" is the subject, the verb should be singular to agree with "set" (the head of the noun phrase):
A set of rules governs society.
In your example, you build a relative clause that can refer to either "set or "rules". Hence, both are possible with a minute shift in meaning.
Singular, the verb agreeing with "set":
...a set of rules which governs socitey
There is a set of rules, and this set governs society.
Plural, the verb agreeing with "rules":
...a set of rules which govern society
There is a set of rules, and these rules govern society.