D. Because Muslim League tended to support Landlordism.

The rejection of the offer to form a coalition in United Provinces (now known as Uttar Pradesh) with the Muslim League by the Congress can be attributed to several factors. One of the key reasons was the Muslim League's tendency to support landlordism, which was not in line with the Congress' ideology and objectives.

1. The Congress already had a majority:
The Congress had a majority in the United Provinces, and forming a coalition with the Muslim League was not necessary for them to establish a government. The Congress had a strong base and was confident in its ability to govern without the need for an alliance with a communal political party.

2. Avoiding association with a communal political party:
The Congress, as a secular and inclusive political party, aimed to represent and work for the interests of all communities and religions in India. The Muslim League, on the other hand, had a communal agenda and primarily worked towards the interests of the Muslim community. The Congress did not want to be associated with a party that focused on one particular religious group, as it would go against their principles of unity and secularism.

3. Difference in political ideologies:
The Congress was dominated by liberals who advocated for social and political reforms, while the Muslim League was relatively conservative and supported the interests of the Muslim landlords. The Congress did not agree with the Muslim League's support for landlordism, which was seen as an exploitative system that perpetuated inequality and hindered social progress.

4. Congress' focus on broader national unity:
The Congress had a broader vision of national unity and aimed to bring together people from all backgrounds and communities to work towards India's independence. They believed in a united and inclusive India where all citizens had equal rights and opportunities. The Congress may have perceived the Muslim League's support for landlordism as a hindrance to achieving this vision of national unity.

In conclusion, the rejection of the offer to form a coalition with the Muslim League by the Congress in the United Provinces was primarily due to the Muslim League's support for landlordism, which was contrary to the Congress' ideology and objectives. The Congress aimed for a secular, inclusive, and united India, and did not want to associate with a party that focused on communal interests and supported an exploitative system like landlordism.

In the United Provinces, the party had rejected the Muslim League proposal for a coalition government partly because the League tended to support landlordism, which the Congress wished to abolish, although the party had not yet taken any concrete steps in that direction.