Answer:
University of North Carolina scientists who study the effects of ageing argue that a combination of genetic and environmental factors control the speed of the ageing process in humans.
Step-by-step explanation:
The best relative pronoun to use in the sentence above is who. This pronoun is used to refer to people. In this case, it refers to "scientists". "Who" is also used when it is the subject of the relative clause, which is the case here - "who" is the subject of "study the effects of ageing". "Whom" would be inadequate because it cannot be the subject, and "which" refers to objects, animals, ideas and so on, but not to people. We can use "that" as well but, since the sentence seems to be a bit more formal, "who" is the best choice.