In the spoken language you don't need 「か」
With this you make a rise in intonation at the end of the sentence to indicate that it is a question. All of these examples below, translate as "Do you want this?"
これを欲{ほ}しい？
This can also be used
これを欲{ほ}しいの？
Of coarse someone just learning would be better to stick to using the polkite form such as this,
これを欲{ほ}しいんですか？
Using something like the below example can across as being rather rude if not said correctly. Sticking to using 「ですか」should be your goal until you know how to properly use it in a sentence.
これを欲{ほ}しいか？
So your example would be fine to say something like this
彼がいつもあそこに立っているのはなぜ？
Why does he always stand over there.