Final answer:
The correct answer is A) lower; last; spin, as orbitals of lower energy are filled first and the last electron's spin is indicated by the direction of the arrow in an orbital diagram.
Step-by-step explanation:
When drawing an orbital diagram, orbitals of lower energy are filled first. By convention, the last electron in a given orbital is designated as ↓ and the direction of the arrow indicates the electron spin. Therefore, the correct answer is A) lower; last; spin. Hund's Rule guides the filling of electrons in orbitals and states that orbitals of equal energy are each occupied by one electron before any orbital is occupied by a second electron, and all such electrons must have the same spin. This helps minimize the repulsive forces between electrons. In orbital filling diagrams, which provide a visual representation of electron arrangements, squares or circles represent orbitals and arrows represent electrons, pointing upwards or downwards to indicate spin directions.