Step 1: Recall the year directly:
1875 A.D. marks: Foundation of the Arya Samaj.
Step 2: Provide brief context:
Swami Dayananda Saraswati founded the Arya Samaj in Bombay in 1875, a reformist Hindu organisation advocating a return to Vedic principles and opposing idol worship, caste rigidity and child marriage.
Final Answer:
\[ \boxed{\text{Foundation of the Arya Samaj}} \]