Question

Bill, age 11, noticed that he has some pubic hair. according to the usual sequence of puberty, his next pubertal event will probably be

Answer 1

The fact that Bill is 11 years old, means that he is in puberty, which is the process of physical changes through which a child's body matures into an adult
Bill, age 11, noticed that he has some pubic hair.
The sequence is:
1) growth of testes, 2) first pubic hairs, 3) enlargement of penis, 4) first ejaculation of seminal fluid (spermarche), 5) height spurt, 6) final pubic-hair growth. 
According to the usual sequence of puberty, his next pubertal event will probably be penis growth.