The correct answer is C. 6 m.
The spacing between phase conductors of a 220 kV line is approximately equal to 6 m. This is because the spacing between phase conductors is determined by the voltage of the line and the type of insulation used. For a 220 kV line, the insulation is typically air, which has a relatively low dielectric strength. This means that the spacing between phase conductors must be relatively large to prevent arcing.
The other options are incorrect because they are either too small or too large. Option A, 2 m, is too small because it would not provide enough clearance to prevent arcing. Option B, 3.5 m, is too large because it would be more expensive to construct and maintain. Option D, 8.5 m, is also too large because it would not be necessary to provide that much clearance to prevent arcing.