The Bombay Secretariat, which was designed in the 1870s, was created by H. St. Clair Wilkins. It is important to note that the Bombay Secretariat refers to the administrative headquarters of the Bombay Presidency during the British rule in India.

Option 2, Sir Cowasjee Jehangir Readymoney, may seem like a plausible choice due to his prominence as a prominent philanthropist and businessman in Bombay. However, there is no historical evidence to suggest that he was involved in the design of the Bombay Secretariat.

Option 3, Purushottamdas Thakurdas, was a prominent industrialist and member of the Indian National Congress, but there is no record of him being involved in the design of the Bombay Secretariat.

Option 4, Nusserwanji Tata, was a renowned Indian entrepreneur and founder of the Tata Group. However, there is no evidence to suggest that he played a role in designing the Bombay Secretariat.

Therefore, the correct answer is option 1, H. St. Clair Wilkins, as he is credited with designing the Bombay Secretariat in the 1870s.