To answer this question, the user needs to have knowledge about Mahatma Gandhi and his relationship with his countrymen.

Here is an explanation of each option:

A. Bhai: While "bhai" is a term of endearment meaning "brother" in Hindi, it was not the name by which Gandhi was lovingly called by his countrymen.

B. Bapu: This is the correct option. Mahatma Gandhi was lovingly called "Bapu" by his countrymen. "Bapu" means "father" in Hindi, and it reflects the respect and admiration that people had for him.

C. Sir: "Sir" is a respectful term used to address someone with authority or in a formal setting. It was not the name by which Gandhi was lovingly called by his countrymen.

D. Dada: "Dada" is a term of endearment meaning "elder brother" in Hindi. While it is used to show affection, it was not the name by which Gandhi was lovingly called by his countrymen.

The correct answer is: B. Bapu