The correct answer is Lord William Bentinck.

During his tenure as Governor-General of India (1828-1835), Lord William Bentinck implemented several significant educational reforms. A key decision was to promote English as the medium of instruction in India. This decision aimed to create a class of Indians educated in Western knowledge and thought, to better facilitate governance and administration. While other Governors-General contributed to the overall evolution of the educational system, Lord William Bentinck is specifically associated with making English the medium of instruction.

• Why other options are incorrect:
• Lord Wellesley focused on expanding British power and influence in India through military and political strategies, not primarily educational reforms.
• Lord Hardinge's tenure focused on other administrative and political issues, not educational reform on this scale.
• Lord Dalhousie's administration was marked by the Doctrine of Lapse, annexations, and other significant political events, with a lesser focus on widespread educational reform of this nature.