I've found this question online. Since it is not a multiple-choice question, I'll answer with my own interpretation of the poem. Feel free to change anything.
Answer and Explanation:
In his poem "No Man is an Island", John Donne talks of our interconnectedness as human beings. According to the poem's speaker, we are all a part of a larger whole. We are but one family, which means what happens to one of us affects all of us. If one man, woman, or child dies, we should mourn and feel for them, since they were a part of our community - no matter where in the world they lived.
Having that in mind, I believe the poet most likely defines friendship as being present and empathetic towards others. Friendship is nothing but the expression of our love and concern for others; it is our understanding of this beautiful connection we have with every human being. To his eyes, a friend is most likely someone you know you can count on to see, accept, understand, and help you.