Final answer:
Blanche is grateful to the doctor because he is a kind stranger, offering her a sense of kindness and courtesy in a moment of vulnerability as he escorts her to a mental institution.
Step-by-step explanation:
The question concerns the character Blanche from the play A Streetcar Named Desire by Tennessee Williams. In the final scene of the play, Blanche expresses gratitude towards the doctor because he represents a kind stranger. Throughout the play, Blanche has faced numerous hardships and betrayals, and at this point, she's lost in her delusions.
The doctor's gentle approach when escorting her to the mental institution gives her a momentary sense of kindness and courtesy that she has been longing for, which is why she says, "Whoever you are—I have always depended on the kindness of strangers." Therefore, the correct answer is 3) Because he is a kind stranger.