To answer this question, let's analyze each option:
Option A) Wood - This option is incorrect because wood does not change color from black to red to grey.
Option B) Iron - This option is incorrect because iron does not change color from black to red to grey.
Option C) Charcoal - This option is correct because when you buy charcoal, it is typically black. When you use it, the charcoal turns red when it is burning. And when you throw it away, it becomes grey ashes.
Option D) Fuel - This option is incorrect because fuel does not change color from black to red to grey.
The correct answer is option C) Charcoal. This option is correct because it matches the description given in the question.