Step 1: Recall the Le Chatelier relation.
For mixtures of combustible gases, the overall lower and upper explosibility limits are estimated using the empirical
\[
\boxed{\text{Le Chatelier relation}.}
\]
Step 2: Identify the correct option.
Since this relation is specifically used for explosive gas mixtures,
\[
\boxed{\text{Le Chatelier relation}}
\]
is the correct answer.
Therefore,
\[
\boxed{(D)}
\]
is the correct answer.