Select the correct alternative to fill in the blank with the correct preposition in the following sentence.
The passionate volunteers worked determinedly ________ environmental conservation and cleaner oceans.

The correct answer is for.

Step-by-Step Explanation:
1. Analyze the Context of the Sentence:
The sentence describes "passionate volunteers" who are putting in dedicated effort regarding "environmental conservation and cleaner oceans." To complete the sentence logically and grammatically, we need a preposition that indicates purpose, support, or direction toward a goal.

2. Understand the Function of the Preposition "for":
The preposition "for" is commonly used to express a aim, target, or cause that someone supports or works to achieve.
• Phrase structure: work for + [a cause/goal]
• Application: Saying that the volunteers "worked determinedly for environmental conservation" correctly indicates that protecting the environment and oceans is the purpose of their hard work.

3. Evaluate the Other Options:
• through: Means going from one end to another or by means of something (e.g., "worked through the weekend"). It does not indicate working toward a cause.
• against: Means in opposition to or against a threat (e.g., "worked against pollution"). Working "against" conservation would imply opposing protection efforts, which contradicts the helpful nature of "passionate volunteers."
• with: Indicates companionship, collaboration, or using a tool (e.g., "worked with a partner"). While volunteers can work with an organization, for is the correct preposition when linking an action to its ultimate objective or cause.

4. Final Sentence:
"The passionate volunteers worked determinedly for environmental conservation and cleaner oceans."