The correct answer is: B. expansion
Expansion is the process of increasing the size or scope of something. In the context of the question, the firm has enjoyed steady expansion in the last ten years. This means that the firm has grown in size, scope, or both.
The other options are not as accurate. Enhancement is the process of making something better or more attractive. Enlargement is the process of making something larger. Extension is the process of making something longer or wider.
In the context of the question, it is not clear how the firm has enhanced itself or made itself more attractive. It is also not clear how the firm has made itself larger or wider. However, it is clear that the firm has grown in size or scope. Therefore, the correct answer is expansion.