In cellular network planning, the total service area is idealised as a mosaic of hexagonal cells. The hexagon is chosen because, among regular shapes that tile a plane without gaps or overlaps (triangle, square, hexagon), the hexagon most closely approximates a circle and covers the area with the fewest cells for a given radius, minimising the number of base stations needed.

To serve such a cell uniformly, the Base Transceiver Station (BTS) is placed exactly at the centre of the hexagon, which is the correct answer. From the centre:

• An omnidirectional antenna radiates equally in all directions, so signal strength falls off symmetrically toward every edge.
• The distance from the BTS to every cell boundary is as even as possible, giving roughly equal path loss and coverage to users at the edges in all directions.
• For higher capacity the cell can instead be sectorised (e.g. 3 or 6 sectors) using directional antennas, but these are still mounted at the central site — this is a centre-excited cell. (In some layouts BTSs are placed at cell corners and shared between cells, but the standard uniform-coverage model uses the centre.)

Why the other options are wrong:

• At any one of the six corners would make coverage lopsided from a single omnidirectional site, leaving the far edge weakly served — corner excitation is a different, directional scheme, not the uniform-coverage default.
• Along the longest edge is geometrically flawed: a regular hexagon has six equal edges, and placing the BTS on an edge again destroys the symmetry needed for uniform coverage.
• Outside the cell boundary would defeat the purpose entirely, since the BTS must be within the cell it serves; being outside would leave the intended area poorly covered.