The correct answer is: A. Sikhism
Sikhism is a monotheistic religion that emphasizes the oneness of God. It was founded in the 15th century by Guru Nanak Dev Ji, who taught that God is beyond all form and description. Sikhs believe in nirguni Bhakti, which is devotion to a divine without Gunas (qualities or form). However, they also accept both nirguni and saguni forms of the divine. Saguni Bhakti is devotion to a divine with form, such as the Hindu deities Vishnu or Shiva.
Shaivism and Vaishnavism are both Hindu sects that emphasize devotion to a particular deity. Shaivism is the worship of Shiva, while Vaishnavism is the worship of Vishnu. Both sects accept both nirguni and saguni forms of the divine.
Therefore, the only sect that emphasizes nirguni Bhakti but accepts both nirguni and saguni forms of the divine is Sikhism.