Hi there, the correct answer to this would be reactivity because color, melting point, and density are all examples of physical properties.
The correct description for an atom of helium would be option C. An atom of helium has its valence electrons in its first energy level, it wouldn't and can't satisfy the Octet rule as it only has 2 electrons, but with 2, it has a full shell, as the first energy level can hold only 2 electrons.
Answer: 127.5ml
Explanation:
To calculate the volume of acid, we use the equation given by neutralization reaction:

where,
are the n-factor, molarity and volume of acid which is 
are the n-factor, molarity and volume of base which is KOH.
We are given:

Putting values in above equation, we get:

Thus 127.5 ml of 0.5M of HNO3 would be needed to react with 85ml of 0.75M of KOH
Structure of a-chloro-b-methylbutyric acid (2-chloro-3-methylbutyric acid is attached below.
The structure was drawn using following information.
1) First, the parent chain was identified. The parent chain is containing four carbon atoms, also, this chain belongs to carboxylic acid as the compound name ends with Butyric acid i.e. butyr (Butane) -ic acid (carboxylic acid).
2) Secondly, the positions of substituents were assigned by using the rule which sates that the numbering in carboxylic acid must satrt from the carbonyl carbon. Therefore, the carbon which is attached to carbonyl carbon is named as alpha (2nd position) and carbon next to alpha carbon is named as beta carbon (3rd position). Hence, we add chlorine atom on 2nd carbon next to carbonyl group and methyl group at 3rd carbon with respect to carbonyl carbon.