Step 1: Identify the founder first.
Satyashodhak Samaj is closely tied to Jyotiba Phule's work among the lower castes of western India, so recalling his timeline helps fix the year.
Step 2: Place it on the reform timeline.
Phule was active from the 1850s onward, and he formally organised the Satyashodhak Samaj in 1873, two years before the Arya Samaj (1875) was set up by a different reformer.
Step 3: Eliminate look alike dates.
1857 belongs to the political revolt, and 1829 belongs to the Sati abolition regulation, so neither fits a caste reform society founded by Phule.
Final Answer:
The correct year is 1873, option (C).