To solve this question, the user needs to have knowledge of the Indian epic, the Mahabharata, and its characters. Specifically, the user needs to be familiar with the five Pandava brothers and their respective roles.

Now, let's go through each option and explain why it is right or wrong:

A. Yudhisthira: This option is correct. Yudhisthira was the eldest of the Pandava brothers and was known for his adherence to dharma (righteousness). He was often referred to as Dharmaraj because of his commitment to upholding moral and ethical values.

B. Bhima: This option is incorrect. Bhima was the second-eldest of the Pandava brothers and was known for his strength and prowess in battle, but he was not referred to as Dharmaraj.

C. Arjuna: This option is incorrect. Arjuna was the third-eldest of the Pandava brothers and was known for his skill as an archer and warrior, but he was not referred to as Dharmaraj.

D. Nakul: This option is incorrect. Nakul was one of the younger Pandava brothers and, like his brother Sahadev, was known for his skill with horses. However, he was not referred to as Dharmaraj.

Therefore, the answer is: A. Yudhisthira