What is the most controlled way to turn a neutral grey into a warm grey?

Correct answer: Add a very small amount of yellow-orange

The governing concept is colour temperature: warm colours tend toward yellow, orange, or red, while cool colours tend toward blue. Adding a very small amount of yellow-orange shifts the neutral grey toward warmth while preserving control and avoiding a strong hue change. More blue cools it, black lowers value, and white only lightens it. Therefore option A is the most precise correction.