The height of transmitting antenna is 180 m and the height of the receiving antenna is 245 m. The maximum distance between them for satisfactory communication in line of sight will be: (given $$R = 6400$$ km)

The maximum distance for line of sight communication:

$$d = \sqrt{2Rh_T} + \sqrt{2Rh_R}$$

Given: $$h_T = 180$$ m, $$h_R = 245$$ m, $$R = 6400$$ km $$= 6400 \times 10^3$$ m.

$$d_T = \sqrt{2 \times 6400 \times 10^3 \times 180} = \sqrt{2304 \times 10^6} = 48 \times 10^3 \text{ m} = 48 \text{ km}$$

$$d_R = \sqrt{2 \times 6400 \times 10^3 \times 245} = \sqrt{3136 \times 10^6} = 56 \times 10^3 \text{ m} = 56 \text{ km}$$

$$d = 48 + 56 = 104 \text{ km}$$

This matches option 4: 104 km.