The correct answer is (c) 1870.
Satyarth Prakash is a book written by Swami Dayanand Saraswati, the founder of Arya Samaj. It was first published in 1870 in Hindi. The book is a treatise on Hinduism and its philosophy. It covers a wide range of topics, including the nature of God, the soul, the universe, and the Vedas. Satyarth Prakash is considered to be one of the most important texts of Arya Samaj.
The other options are incorrect. Option (a) is incorrect because Satyarth Prakash was published in 1870, not 1890. Option (b) is incorrect because Satyarth Prakash was published in 1870, not 1880. Option (d) is incorrect because Satyarth Prakash was published in 1870, not 1875.