The threshold markings are A. 4 m wide B. 1 m clear space between adjacent C. Placed symmetrically on either side of the runway centre line D. All the above

The correct answer is D. All the above.

The threshold markings are 4 m wide, with a 1 m clear space between adjacent markings. They are placed symmetrically on either side of the runway center line.

The threshold markings are used to indicate the beginning of the runway. They are important for pilots to be aware of, as they need to make sure that they are landing on the runway and not beyond it.

The threshold markings are typically white or yellow, and they are made of a durable material that can withstand the wear and tear of aircraft tires.

The threshold markings are an important part of the runway safety system, and they help to ensure that aircraft land safely.