The correct answer is C. 80 cm.
The spacing of timber piles is determined by a number of factors, including the type of soil, the load-bearing capacity of the piles, and the desired depth of the foundation. In general, the spacing of timber piles should be between 80 and 100 cm. However, it is important to consult with a structural engineer to determine the specific spacing requirements for a particular project.
Option A (50 cm) is too close together, and the piles would not be able to support the load. Option B (60 cm) is a bit closer than ideal, but it may be acceptable in some cases. Option C (80 cm) is the most common spacing for timber piles. Option D (90 cm) is too far apart, and the piles would not be able to provide adequate support.
It is important to note that the spacing of timber piles is just one factor that affects the stability of a foundation. Other factors, such as the type of soil, the load-bearing capacity of the piles, and the depth of the foundation, must also be considered. It is always best to consult with a structural engineer to ensure that a foundation is designed to meet the specific needs of a project.