The correct answer is: C. 10 cm.
A cavity wall is a type of wall that consists of two layers of masonry with a hollow space between them. The hollow space is filled with insulation, which helps to improve the thermal performance of the wall. The width of the hollow space is typically between 5 and 15 cm, but it should not exceed 10 cm in order to ensure that the wall is structurally sound.
Option A: 5 cm is too narrow. A cavity wall with a width of 5 cm would not provide enough insulation to be effective.
Option B: 7.5 cm is a common width for a cavity wall. However, it is important to note that the width of the hollow space should not exceed 10 cm in order to ensure that the wall is structurally sound.
Option C: 10 cm is the maximum width for a cavity wall. This is because a wider hollow space would make the wall more susceptible to damage from wind and other forces.
Option D: 15 cm is too wide. A cavity wall with a width of 15 cm would be more susceptible to damage from wind and other forces.