Question

I ---- her for a long time (know, have known, had known) ​

Answer 1

The answer must be I For a long time.

Answer 2

I have known her for a long time.

• The sentence is in present perfect tense.
• This tense expresses anything which proceeds from the past to the present.
• For example: Tom and Rose have known each other since 2019 or  I have known Carl since I was a little boy.
• In the given sentence, the person has already known the other person since a long time.
• Thus, the correct sentence is - I have known her for a long time.

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