Alwar saints were a group of Vaishnavite saints in South India who lived between 5th and 9th century CE. They were known for their devotional poetry and devotion to Lord Vishnu. However, the correct answer to the given question is option 'D' - Shiva. Let's understand why.

Explanation:

1. Alwar Saints and Shaivism:
- Although the Alwar saints were primarily devoted to Lord Vishnu, some of them also expressed their devotion to Lord Shiva.
- Some scholars even argue that the Alwar saints were not exclusively Vaishnavites but also had a strong Shaivite influence.
- For example, Nammalvar, who is considered the first of the Alwar saints, composed several hymns in praise of Lord Shiva, besides his devotion to Lord Vishnu.

2. "Tevara" and Shaivism:
- Another reason why the Alwar saints worshipped Lord Shiva is the tradition of "Tevara" in Tamil Nadu.
- Tevara is a form of devotional poetry that emerged in the Tamil-speaking region in the 7th-8th century CE and is associated with the Shaivite Bhakti movement.
- The Alwar saints were contemporaries of the Tevara poets and were influenced by their devotional fervor and poetry.
- Some of the Alwar saints, such as Sundarar and Manickavasagar, were themselves Tevara poets and composed several hymns in praise of Lord Shiva.

3. Alwar Saints and the Bhakti Movement:
- The Alwar saints were part of the larger Bhakti movement that swept across India between the 7th and 12th century CE.
- The Bhakti movement was characterized by the rejection of caste and ritualistic practices and emphasized the importance of personal devotion and direct experience of God.
- The Alwar saints, through their devotional poetry, sought to establish a direct and intimate relationship with God, whether it was Vishnu or Shiva.

In conclusion, while the Alwar saints were primarily Vaishnavites, they also expressed their devotion to Lord Shiva, either through their own poetry or through the influence of the Shaivite Bhakti movement. Therefore, the correct answer to the given question is option 'D' - Shiva.