Step 1: Identify the work.
Satyarth Prakash (The Light of Truth) is a seminal work that sets out the principles of the Arya Samaj and Vedic teachings.
Step 2: Attribute authorship.
This text was authored by Swami Dayanand Saraswati, the founder of the Arya Samaj. Hence option (D) is correct.
Step 3: Eliminate distractors.
Gokhale, Jinnah, and Mahatma Gandhi did not write Satyarth Prakash.