D) Volume
All other answers involve chemical reactions and are hence, not physical properties.
Answer:
a. NH₃ : base
CH₃COOH (acetic acid) : acid
NH₄⁺ : conjugate acid
CH₃COO⁻ : conjugate base
b. HClO₄ (perchloric acid) : acid
NH₃ : base
ClO₄⁻ : conjugate base
NH₄⁺ : conjugate acid
Hope this helps.
The Calcium ion is an Alkaline earth metal and wants to give up the 2 s orbital elections and become a +2 cation.
Oxygen has six valence electrons and is looking to gain two electrons to complete the octet (8) electron count in the valence shell making it a -2 anion.
When the charges of the Calcium +2 and the Oxygen -2 are equal and opposite, the ions for an electrical attraction. (Remember Paula Abdul told us "Opposites Attract")
This one to one ratio of charges makes the formula CaO