Answer: it is a direct variation.
Justification:
In a direct variation the variables are related by a proportionality constant in this way:
y = k x
Tthan means that the value of y is always the product of a constant times the value of x.
The situation discribed for the coal may be written as:
number of tons of coal burned: c
number of hours: h
⇒ c = k × h
Which is that you can calcualte the number of tons of coal burned at any time, once you know the proportionality constant k.
