- Statement i is correct as it states that Arab merchants established trade links along the western coast of the Indian subcontinent from early centuries CE, which indeed facilitated the introduction of Islam to that region.
- Statement ii is incorrect because while the Delhi Sultanate was established by Turks and Afghans in the 13th century, Islamic rulers generally respected the religious institutions of non-Muslim subjects, as noted in the provided content.
- Statement iii is correct; Muslims in the subcontinent adhered to fundamental Islamic practices while also integrating local customs, creating a unique blend of religious and cultural practices.
- Statement iv is incorrect because mosques in the subcontinent do display regional variations in architectural styles, demonstrating a synthesis of universal Islamic elements with local traditions.
Thus, the correct statements are i and iii, making option A the correct answer.