Which "big five" personality trait refers to the degree to which a person seeks to be with others, to behave in an outgoing manner, and generally to be sociable?

The answer is Extroversion.

In the big 5 theory of personality, extroversion (often known as extraversion) is one of the five core traits believed to make up human personality. Extroversion is characterized by sociability, talkativeness, assertiveness and excitability. People who are high in extroversion tend to seek out social stimulation and opportunities to engage with others. These individuals are often described as being full of life, energy and positivity. In group situations, extroverts (extraverts) are likely to talk often and assert themselves.

using the scientific method.