The correct answer is A. circular.
A circular central island is the most suitable shape for an intersection of two equally important roads that cross roughly at right angles. This is because a circular island provides the most efficient use of space and allows for the smoothest flow of traffic.
A circular island is also the safest shape for an intersection, as it reduces the risk of head-on collisions. Additionally, a circular island is the most aesthetically pleasing shape, as it creates a sense of order and symmetry.
The other options are not as suitable for an intersection of two equally important roads that cross roughly at right angles. A elliptical island would not provide as efficient use of space, as it would be longer than a circular island. A tangent island would not allow for the smoothest flow of traffic, as it would create a sharp turn for drivers. A turbine island would not be as safe as a circular island, as it would increase the risk of head-on collisions.
In conclusion, the correct answer is A. circular.