What part of speech is "and"?

Why it's time to lay the stereotype of the 'teen brain’ to rest 2. part b: which detail from the text best supports the answer to part a? a “brain deficits don’t make teens do risky things; lack of experience and a drive to explore the world are the real factors.” (paragraph 1) b “according to this theory, the prefrontal cortex, the center of the brain’s cognitive-control system, matures more slowly than the limbic system, which governs desires and appetites including drives for food and sex.” (paragraph 5) c “in experiments that mimic the well-known marshmallow test, in which waiting for a bigger reward is a sign of self-control, adolescents are less impulsive than children and only slightly more so than adults.” (paragraph 12) d “the majority of adolescents do not die in car crashes, become victims of homicide or suicide, experience major depression, become addicted to drugs or contract sexually transmitted infections.” (paragraph 14)