In which situation is written confirmation after an immediate phone call most necessary?

Correct answer: Setting important deadlines and responsibilities

The governing concept is selecting complementary communication methods. A phone call provides immediate interaction and clarification, while written confirmation creates a reliable record of what was decided. Deadlines and responsibilities can be misunderstood or disputed later, so recording them is important. Casual greetings, weather talk, or a picture preference normally do not need formal documentation. Therefore, option D is the most appropriate answer.