Step 1: Understanding the Question:
The question asks to identify the social reformer of Kerala who is famously associated with the progressive slogan: ``One caste, one religion, one God for mankind''.
Step 2: Key Concepts and Social Reforms in Kerala:
During the late 19th and early 20th centuries, Kerala was deeply plagued by caste rigidity, untouchability, and social inequalities.
Several socio-religious reform movements arose to challenge these dogmas and promote universal human equality.
Step 3: Detailed Explanation:
• Sree Narayana Guru: He was a revolutionary spiritual leader and social reformer of Kerala.
- He led a reform movement against the injustice in the caste-ridden society of Kerala in order to promote spiritual enlightenment and social equality.
- His most famous Malayalam proclamation was: ``Oru Jathi, Oru Matham, Daivam Manushyanu'' which translates to ``One Caste, One Religion, One God for Man''.
- He consecrated a Shiva temple at Aruvippuram in 1888, challenging the monopoly of upper-caste priests.
• Other Reformers:
- Vakkom Moulavi worked for the social and educational upliftment of Muslims.
- Ayyankali fought for the rights of lower-caste Dalits, specifically focusing on education and freedom of movement.
- Thycaud Ayya was a prominent guru who initiated inter-dining reforms.
Step 4: Final Answer:
Therefore, this universal slogan is associated with Sree Narayana Guru, corresponding to Option (D).