Your second one is correct:
Some people contend that measures can be undertaken that yield positive outcomes.
Yield here agrees with measures. There might be some confusion because the relative clause that yield positive outcomes does not come directly after the noun phrase that it modifies. If we place it directly after (adjacent to) that noun phrase, we get
Some people contend that measures that yield positive outcomes can be undertaken.
It should be clearer now that yield agrees with measures.
Relative clauses do not have to immediately follow the head noun that they modify. A relative clause that does not follow immediately the head noun is called an extraposed relative clause.
Cambridge University's Language and Cognition,
Volume 9, issue 2,
contains an abstract of a study called 'When relative clause extraposition is the right choice, it’s easier', and it explains that
speakers prefer [relative clause exposition] over adjacent ordering when the [relative clause] is long in relation to the [verb phrase], the subject [noun phrase] is indefinite, and the main verb is passive/presentative.
Let's check the clause which concerns us, which is
measures can be undertaken that yield positive outcomes
1 is the relative clause long in relation to the verb phrase?
It is longer. The verb phrase is can be undertaken; the relative clause is that yield positive outcomes.
2 Is the subject indefinite?
Yes, measures is indefinite. We can't identify which measures are being referred to. Perhaps no definite measures are in the speaker's mind.
3 Is the main verb is passive/presentative?
It is the passive can be undertaken.
The same abstract contains another example of relative clause exposition:
Some options were considered that allow for more flexibility.
Notice how similar in structure it is to the one you ask about.