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The book 'English Grammar in Use, Murphy' contains the following exercise under the Present perfect/Present perfect continuous chapter:

• Are you OK? You look as if ____ (you / cry).

I have not found any explanations why 'have been crying' is the correct answer.

Based on my understanding, the action has already been completed (the person has cried and now we're asking her by focusing on the result of the action). Other words, I can't imagine a situation when we're asking someone who's crying right now by focusing on the activity and not the result.

Would anyone explaining the Present Perfect Continuous choice here?

If the action referred to by the lexical verb in question (e.g. cry, swim, paint) refers to something that does not have a goal or end-point but is an atelic process or activity, then when that verb is coupled with the perfect we use the continuous:

You're all wet. You look as if you have been swimming.

We wouldn't say:

You're all wet. You look as if you have swum. ^No

at least not with the past participle swum all by itself like that. You would need to add something that creates a sense of completion:

I've never seen a person who looked as wet as you do. You look as if you have swum across the Atlantic Ocean.

P.S. That the action has ceased does not mean that it had a goal, a pre-determined end-point, to begin with. There is a difference, for example, between swim and swim across. That you have stopped swimming does not mean that you have reached a goal; you have merely ceased the activity.