Norman Triplett observed that adolescents wound a fishing reel faster in the presence of someone working simultaneously on the same task. This best illustrates

social facilitation

Explanation:

The concept of social facilitation was first proposed by Norman Triplett. After observing a cycling event, in which he noticed that cyclist performed better when they have others around competing with them, rather than cycling alone just against time. This led him to duct an experiment where he gave some children strings to wound fishing reel. He observed that, children wound the fishing reel faster in the presence of other children performing similar task, when compared to when perform such task alone.

This concept discovered by Norman Triplett, best illustrates the concept of social facilitation, which posits that, individuals tend to perform better in the mere presence of others or when they perform a task with others, than when performing the task alone.