The smallest particle into which an element can be divided and still have the properties of that element is called an atom.

D)Pulsars are easy to identify by their almost perfectly steady periods of pulsation.

Explanation:

Neutron stars are the stars which do not have enough mass to turn into a black hole. They have very high density of mass. As the star core collapses as the neutron star is smaller in size the speed at which it rotates increases. This happens as the angular momentum is conserved. Some neutron stars eject electromagnetic radiation from their poles. The ejection takes place in precise intervals of time.

This makes it very easy in order to locate the position of one's planet in the universe.

So, the scientists chose pulsars as they are easy to identify by their almost perfectly steady periods of pulsation.