Directions: Choose the appropriate preposition to complete the sentence.

The young diplomat was highly competent ________ resolving sensitive disputes.

The correct option is at.

Explanation:
The adjective competent means having the necessary ability, knowledge, or skill to perform a task successfully.

When connecting competent to a specific activity or action represented by a gerund (a verb ending in -ing, such as resolving), the preposition at is used to indicate proficiency in carrying out that particular action.

Let's look at why the other prepositions are not appropriate here:
• for: Used to denote purpose, destination, or duration, not proficiency in performing an action.
• in: Typically used with competent when referring to a broad sphere, field, or subject area (e.g., "competent in legal matters"), whereas at is preferred when referring directly to a specific action or skill.
• with: Used to indicate tools, instruments, or general handling of items/people (e.g., "competent with computers"), rather than an active verb phrase.

Therefore, the complete and grammatically correct sentence is:
"The young diplomat was highly competent at resolving sensitive disputes."