A. I am from Korea. ​(Correct)

I am from the U.K --> I am from U.K.

=> Corrected: The sentence is corrected grammatically.​

Where are you from? ​(Correct)​

=> Corrected capitalization. 

B. Thank you. Have a nice day. (Correct)

=> Corrected punctuation.​

Great! --> Bye!​

Mom, what he do job? --> Mom, what's his job?

=> Corrected: The sentence is corrected grammatically.​​