Explanation:

- Sentence and Requirement: She was known for her ability to communicate effectively and express herself clearly in writing and speaking. The task is to find a one-word substitute.

- Option 1: Incoherence – This means lacking clarity or organization in thought or speech. It is the opposite of clear communication.

- Option 2: Articulation – This refers to the ability to express thoughts clearly and effectively. It matches the underlined requirement perfectly.

- Option 3: Mumbling – This means speaking in a low, unclear manner. It is not related to clear expression.

- Option 4: Rambling – This means talking or writing in an aimless or lengthy way, not focused on clarity.