James is reading an erotic story in a magazine when he notices his respiration and heart rate increasing. He is also experiencing penile erection. Which phase is James currently experiencing?

He is experiencing puberty.

Explanation:

In simple words, puberty refers to the phase in which boys and girls become sexually mature. It usually starts around age 12 to 14 (although it can occur a few years sooner) and also implies physical changes.

Among other things, boys develop a higher production of testosterone, the principal male sex hormone. That would explain why James is experiencing these changes while reading the story.