Which with two or more options?

[Loriko]

In the English text book, it mentions that
the word "which" is used in the setence with two or
more than two options attached to the question sentence.
For example, in the sentence
" Whcih do you want to drink, coffee or juice?"there are two options in it.
But I find out that there are some sentences with no
options attacted to the question sentence beginning with
the word "which".
For instance,
"Which day does Ryan hate?"
"Which dog is Lucy?"
"Which girl has long hair?"
"Which is Peter's cellphone?"
However, there is a question printed on the exam paper:
A: "____" sport does Mary Love?
B: She loves basketball.
The answer given by the answer sheet is "What" not "Which."
If we can use "which" in a sentence with no options, then why can this
sentence's answer be the word " Which?"

[Beeesneees]

Hi Loriko,

'Which' has options, but they may not be given in the same sentence as the question.
In your examples the options would have been elsewhere in the wider context.

Which day does Ryan hate? - choose from a particular set of days
What day does Ryan hate? - out of all the days that have ever existed

Which dog is Lucy's? - choose from a particular set of dogs
What dog is Lucy's? - out of all the dogs in existence
_________________
Cheers m' dears!

[Thredder]

As BN says, 'which' here is kind of short for 'which of these', even if the 'these' isn't explicit in the sentence, Which [of these dogs] is Lucy's?