Language immersion is one of the best ways to improve English pronunciation, intonation, and usage. It is especially helpful for young learners. At the same time, grammar drills is also important for learning the basic grammatical rules and sentence patterns. Find a teacher who can help students with both would be the best option.

If you know the basic English, I think chatting is the best option. But you should find an English teacher too. Chatting is good to practice but a teacher is really important to learn the rules and so on. ^^

I choose D as the best answer. D. B & C

I think chatting with a native speaker is the best way to learn. He /she can correct any mistakes you make, and tell you how to say it correctly.