Final answer:
Element X, having a completely filled outer electron shell, is chemically stable and is possibly a noble gas with high electronegativity, as these elements are characterized by their filled valence shells that make them inert and Option a & d, unreactive.
Step-by-step explanation:
If a student observes that element X has a completely filled outer electron shell, two conclusions can be drawn: a) Element X is chemically stable and less likely to form chemical bonds and d) Element X is a noble gas and has high electronegativity. Noble gases, such as helium, neon, and argon, are known for having their outermost shells completely filled with electrons, which makes them very stable and inert, meaning that they do not readily form chemical bonds. They have large ionization energies and are unreactive due to their stable electron configurations.