Final answer:
The Full block style is considered the most efficient letter format for the ambulatory care setting due to its organized structure and ease of readability.
Step-by-step explanation:
The letter style considered most efficient for the ambulatory care setting is the Full block style. This format is very organized, which makes information easy to find and read quickly, a vital aspect for fast-paced medical environments. Full block style aligns all text to the left margin, is single-spaced within paragraphs, double-spaced between paragraphs, and requires no indentation at the start of paragraphs. A clear, readable font such as size 12 Times New Roman or Arial is recommended when using the full block style.