B

Step 1: Understand the concept of mobility in semiconductors. Mobility ($\mu$) is defined as the ability of charge carriers (electrons and holes) to move through a material when subjected to an electric field. It is influenced by the effective mass of the carriers, their scattering mechanisms, and their charge.
Step 2: Compare electrons and holes. Electrons are negatively charged carriers, while holes represent the absence of an electron and are considered as positively charged carriers. However, the key factor affecting mobility is not the charge itself, but the effective mass of the carriers.
Step 3: Electrons have a lower effective mass compared to holes. This means that they can accelerate more quickly when an electric field is applied, leading to higher mobility. The effective mass reflects how a particle's motion behaves in a crystal lattice.
Step 4: Therefore, the greater mobility of free electrons compared to free holes is primarily due to their lighter effective mass, which allows them to move faster. Thus, the correct answer is Option B: They are light.