Concept:
The national language of a country is usually established by its constitution and reflects the cultural identity of the vast majority of its population. The language movement of 1952 played a foundational role in the historic emergence of Bangladesh.
Step 1:
The national and official language of Bangladesh is Bengali (also locally known as Bangla). It is spoken by more than 98% of its population.
Step 2:
Analyzing other choices: Urdu is the national language of Pakistan, Hindi is an official language of India, and Rangpuri is a regional language/dialect spoken in parts of Bangladesh and India, but it is not the national language.