Exercício: usar "did" ou "have"

Hi everybody! I'm new here, so I appreciate any help from you!

I've got the following question..

If I want to say "Quando Homero começou trabalhar na Planta de Energia Nuclear?"

Which of the following sentences is the correct/best sentence among the options and why.

a) When have Homer started working in the Nuclear Power Plant?
b) When did Homer start working in the Nuclear Power Plant?
c) When did Homer start to work in the Nuclear Power Plant?

Thanks in advance! I really want to improve my english and I need some help this time..
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a)When have Homer started working in the Nuclear Power Plant? (incorrect)
You should not use the present perfect to ask for an action that happened at a specific time in the past. Also, "Homer" is third person singular.

b) When did Homer start working in the Nuclear Power Plant?
c) When did Homer start to work in the Nuclear Power Plant?
Sentences b and c are both correct. The verb "to start" can be followed by gerund or infinitive with no change in meaning, in this sentence.

http://www.englishpage.com/gerunds/geru ... t_list.htm
http://www.italki.com/question/105963