The correct answer is D. All of the above.
L-shaped walls, T-shaped walls, and counterfort type retaining walls are all suitable for heights beyond 6 m.
L-shaped walls are a type of retaining wall that is made up of two legs that are joined at a right angle. The legs are typically made of concrete or masonry, and the backfill is held in place by the weight of the wall. L-shaped walls are often used in areas where there is a lot of lateral movement, such as on slopes or near rivers.
T-shaped walls are similar to L-shaped walls, but they have an additional leg that extends from the back of the wall. This additional leg helps to distribute the load of the wall more evenly, and it also makes the wall more stable. T-shaped walls are often used in areas where there is a lot of soil pressure, such as on steep slopes.
Counterfort type retaining walls are made up of a series of vertical piers that are connected by horizontal beams. The piers are typically made of concrete or masonry, and the beams are typically made of steel. The backfill is held in place by the weight of the wall and the friction between the backfill and the wall. Counterfort type retaining walls are often used in areas where there is a lot of lateral movement and soil pressure, such as on steep slopes or near rivers.
All of these types of retaining walls are suitable for heights beyond 6 m, and the best type of wall to use will depend on the specific conditions of the site.