Introduction:
The Nayanars were a group of 63 Tamil Shaivite saints who lived between the 5th and 10th centuries CE. They were devoted to Lord Shiva and composed hymns in praise of him. One of the most famous Nayanars is Appar, who is also known as Tirunavakkarasu.

Explanation:
Appar, also known as Tirunavakkarasu:
- Appar is popularly known as Tirunavakkarasu and is considered one of the foremost Nayanars.
- He was born in the 7th century CE in a Brahmin family in the present-day Thiruvamur, Tamil Nadu.
- Initially, Appar was a staunch follower of Jainism. However, after witnessing a miracle performed by Lord Shiva, he converted to Shaivism.
- Appar traveled extensively to various Shiva temples and composed hymns in praise of Lord Shiva.
- He faced numerous challenges and obstacles in his spiritual journey, including persecution by the Pallava king Mahendravarman I.
- Appar's devotion and steadfastness in the face of adversity made him a revered figure among the Nayanars.
- His hymns, known as Tevaram, are still recited in Shiva temples in Tamil Nadu.
- Appar's contributions to Tamil literature and devotion to Lord Shiva earned him the title of Tirunavakkarasu, meaning "king with a beautiful tongue."

Other Options:
- Nammalvar, also known as Maran, was a Vaishnavite saint who composed the Tiruvaymozhi hymns in praise of Lord Vishnu. He is not the same as Appar.
- Nakkeerar was a renowned poet and scholar, but he was not one of the Nayanars.
- Sambandar, also known as Thirugnana Sambandar, was one of the 63 Nayanars. He composed hymns in praise of Lord Shiva from a very young age.

Conclusion:
Among the given options, Appar is popularly known as Tirunavakkarasu and is considered one of the most prominent Nayanars. His devotion to Lord Shiva, extensive travel, and composition of hymns in praise of the deity have made him a revered figure in Tamil Nadu.