Question

According to the wave-mechanical model of the atom, an orbital is a region of the most probable location of

Answer 1

 That would be (3) an electron

Answer 2

With the advancement of science, electrons seemed to possess both particle and wave nature. 
this is called the dual nature where electrons have both particle and wave properties. 
earlier it was believed that electrons used to orbit around the nucleus in orbits.
Later it was found that electrons do not have fixed positions, the exact momentum and position of electrons cannot be determined at the same time 
therefore the wave mechanical model was introduced which introduced orbitals.

Orbitals are spaces in which electrons are most likely to be found. These regions have the highest probability of an electron being found here.

correct answer is 3) an electron