Answer:

Introduction:
Modal auxiliaries are a type of helping verbs that express the speaker's attitude or opinion towards an action. They are used to indicate possibility, necessity, ability, permission, and obligation. In this question, we need to choose the correct modal auxiliary to convey the sense of obligation.

Explanation:
To determine the correct modal auxiliary, let's analyze the options given:

a) Ought: "Ought" expresses a moral obligation or duty. It is used to indicate that something is the right thing to do. Example: "We ought to help those in need." In the given sentence, the sense of obligation is present, so "ought" seems to be a suitable choice.

b) Should: "Should" is used to express advice, recommendation, or expectation. It suggests that something is the right thing to do based on societal norms or personal judgment. Example: "You should study for the exam." While "should" also conveys a sense of obligation, "ought" is a better fit in this context as it emphasizes moral duty.

c) Must: "Must" is used to express a strong obligation or necessity. It indicates that something is mandatory or required. Example: "You must submit the assignment by tomorrow." While "must" conveys a sense of obligation, it implies a stronger requirement than what is needed in the given sentence.

d) Could: "Could" is used to express possibility or ability. It suggests that something is potentially achievable or permissible. Example: "We could go for a walk in the park." While "could" indicates a possibility, it does not convey the necessary obligation to keep the surroundings clean.

Conclusion:
Among the given options, the correct modal auxiliary to fill in the blank is "ought." This choice effectively conveys the sense of obligation to keep our surroundings clean.

Should