According to IS: 875 Part 3, the desiring wind speed is obtained by multiplying the basic wind speed by factors (k1, k2) and (k3). The correct answer is option 'C', which states that (k3) represents the topography factor. Let's break down the explanation in detail:

Basic Wind Speed:
The basic wind speed is the characteristic wind speed for a specific location, which is determined based on meteorological data. It represents the average wind speed that can be expected in that area.

Factors affecting Wind Speed:
To determine the desiring wind speed, the basic wind speed is multiplied by several factors. These factors account for various parameters that influence the wind speed at a particular location. The factors are as follows:

1. (k1, k2) - Terrain Height Factor and Structure Size Factor:
The first set of factors, (k1, k2), accounts for the influence of terrain height and structure size. The terrain height factor (k1) takes into consideration the effect of the height of the land where the structure is located. The structure size factor (k2) considers the size and shape of the structure itself.

2. (k3) - Topography Factor:
The second set of factors, (k3), represents the topography factor. This factor accounts for the influence of the surrounding topography on the wind speed. Topography refers to the physical features of the land, such as hills, valleys, or slopes. These features can affect the wind flow patterns, causing variations in wind speed.

Importance of the Topography Factor:
The topography factor (k3) is crucial in determining the wind speed for structural design. Different topographical features can create wind acceleration or deceleration, resulting in higher or lower wind speeds than the basic wind speed. By considering the topography factor, engineers can design structures that can withstand the maximum wind loads expected at a particular location.

Conclusion:
According to IS: 875 Part 3, the desiring wind speed is obtained by multiplying the basic wind speed by factors (k1, k2) and (k3). The correct answer, option 'C', states that (k3) represents the topography factor. This factor accounts for the influence of the surrounding topography on the wind speed and is essential for designing structures that can withstand the wind loads in a specific location.