Which symbol on a regulatory marker is used to warn of a rock or other underwater hazard?

Diamond shaped regulatory marker.

Explanation: The regulatory markers are those symbols that are used in order to guide us about any particular place. It warns about the area, how the situation could be, how dangerous it could be, and what are the things that we are not suppose to do. So these informations must be carefully noticed and obeyed.

In order to warn about a rock or any other underwater health hazardous situation, a diamond-shaped marker is generally used in which the diamond shape is made of orange color and the writings are done in black ink.

the answer is true when observing or determining population size, a direct observation is use, in which each organism is counted in a population or in an area. one benefit of this measure is that you get the exact population count but it will take a long time to  determine  the  size  of a  population.