Sodium - metallic bonding
Oxygen - covalent bonding
Sodium chloride - ionic bonding
Magnesium oxide - ionic bonding
Nitrogen fluoride - covalent bonding
Aluminium - metallic bonding
Calcium hydroxide - ionic bonding
Silicon fluoride - covalent bonding (not 100% sure on this one)
Chlorine - covalent bonding
Potassium oxide - ionic bonding
Answer:
The first one.

Explanation:
When comparing two fractions with variables like this, it's important to get to the same denominator in order to compare apples with apples and then be able to do not only comparisons but also perform additions/subtractions.
Question is which denominator to use and how to reach it.
In this case, the question and the answer choices do the work for you. The question asks which one is the LEAST common denominator, and the answers show denominators x² and 4x². The smallest of these is x², however, we can't simplify the first fraction to get to the x² denominator, so we'll go for the 4x².
So, the first fraction has already the correct denominator (4x²), we just have to transform the second one.
We multiply it by 1, expressed in a different way. Since we're multiplying by one, we're not affecting the value, just the way it looks.
Let's do it!, to get the denominator to go from x² to 4x², we need to multiply it by 4... so we'll multiply by 4/4 (which is 1, neutral for the multiplication).

And now you have both fractions on the same denominator, without having changed their value, just their looks
The major carbon sources (reservoirs that release more carbon than absorbed) are: deforestation and fossil fuels
The major carbon sinks (reservoirs that absorb more carbon than released) are:
soil, oceans and plants.
When scientists calculate the amount of carbon dioxide is returned to the atmosphere vs the released the amount of carbon, a large amount is unaccounted for and the total does not add up. This is why scientists believe there is an undiscovered carbon sink somewhere.
The symbol is U and its atomic number is 92