In ancient India, different systems of religious beliefs and sects came into existence. The mention of Saiva sect question refers to one such system that worships the deity Shiva.
Option-1 "Ajivika" was an ancient philosophical and ascetic movement of the Indian subcontinent. They were contemporaries of the early Buddhists and historical Jains, with Ajivikas possibly having influenced the philosophical methodologies of Gandhara Buddhist school but it was not a Saiva sect.
Option-2 "Mattamayura" was indeed a Saiva sect. It is a sectarian tradition within Shaivism, one of the major branches of Indian religion. Hence, it is correctly identified as the answer.
Option-3 "Mayamata" is an ancient Indian treatise on housing architecture and iconography, not a Saiva sect by itself.
Option-4 "Isanasivagurudevapaddhati" is not a sect but a religious text that provides insights into Shaiva Siddhanta rituals and practice.
Therefore, only option-2 "Mattamayura" fits the description of being a Saiva sect in ancient India.