Option 1: Bal Gangadhar Tilak was a prominent leader in the Indian independence movement. However, he was not involved in starting a society for the encouragement of widow remarriage in Maharashtra in 1866.

Option 2: Jyotirao Phule was a social reformer who worked towards improving the rights and education of women and lower castes. While he supported and advocated for widow remarriage, he did not start a specific society for it in Maharashtra in 1866.

Option 3: Vishnushastri Pandit is the correct answer. He was a social reformer who started a society for the encouragement of widow remarriage in Maharashtra in 1866. His efforts were aimed at addressing the social stigma attached to widowhood and promoting the welfare of widows.

Option 4: Pandita Ramabai was a social reformer and women`s rights activist who worked for the empowerment and education of women. However, she did not start a society for the encouragement of widow remarriage in Maharashtra in 1866.

In conclusion, Vishnushastri Pandit is the social reformer who started a society for the encouragement of widow remarriage in Maharashtra in 1866.