I got an A+,

- Congratulations! I'm so happy for you.

(What is the meaning of 'for' in the last sentence? What is the similar sentence, #1 or #2?)

1. I'm so happy thanks to you.
2. I'm so happy about you.

Does that mean:" I'm so happy for you to get A+"?

Does that mean:" I'm so happy for you to get an A+"?

Yes, your interpretation is correct. None of the 3 sentences means the same as the others, though.

In the sentence "I'm so happy for you," the word "for" is used to indicate that the happiness is directed towards the person who got an A+. It means that the speaker is pleased and joyful because of the achievement or success of the other person.

Between the two similar sentences, the more similar one to "I'm so happy for you" is sentence #2: "I'm so happy about you." Both sentences express happiness and positive feelings towards the person in question. However, "for" is commonly used to express a direct connection or cause, while "about" is used to denote a more general feeling of happiness or excitement regarding someone or something. In this case, since the speaker is specifically happy because of the other person's success, "for" is more appropriate than "about."