<u>Answer:</u> The correct answer is the mass number of the most common isotope of the element is 24.
<u>Explanation:</u>
We are given:
An element having atomic number 12 is magnesium and atomic mass of the element is 24.305
The image corresponding will be 
The number '24.305' is the average atomic mass of magnesium element.
Average atomic mass is defined as the sum of masses of each isotope each multiplied by their natural fractional abundance.
Formula used to calculate average atomic mass follows:

Average atomic mass of magnesium = 24.305 amu
As, the average atomic mass of magnesium lies closer to the mass of Mg-24 isotope. This means that the relative abundance of this isotope is the highest of all the other isotopes.
The 'Mg-24' isotope is the most common isotope of the given element.
Hence, the correct answer is the mass number of the most common isotope of the element is 24.
I can't fully answer this question because it is incomplete. In order for me to help you, I could just define what a precipitation reaction is and give a concrete example.
A precipitation reaction consists of two aqueous solutions that when reacted together, forms an insoluble salt. For example,
AgNO₃ (aq) + HCl (aq) --> AgCl (s) + HNO₃ (aq)
In this case, the precipitate is AgCl, Silver Chloride, which appears as a white solid.
Answer:
5.41 ×10⁻²²
Explanation:
We were told right from the question that both the Zinc ions and the Zinc oxide adopts a face-centered cubic arrangement.
Then, the number of ZnO molecule in one unit cell = 4
The standard molar mass of ZnO = 81.38g
Avogadro's constant = 6.023 × 10²³ mole
∴
The mass of one unit cell of zinc oxide can be calculated as:
= 
= 5.40461564×10⁻²²
≅ 5.41 ×10⁻²²
∴ The mass of one unit cell of zinc oxide = 5.41 ×10⁻²²