Answer: Option C. -> 2,2-Dimethylpropane
:
C
$C_5{H}_{12}$ is of the form $Cn{H}_{2n+2}$.
Hence it is a saturated alkane. Further, it is given that it has a quaternary carbon. Hence its structure is:

Longest chain = 3 carbons $⇒$ word root = prop
Primary suffix = ane
Secondary prefix = 2,2-Dimethyl
IUPAC name is (c) 2,2-Dimethyl propane

Answer: Option C. -> Cyclohexane-1,2-dicarbonitrile
:
C
Here, we know that the priority is functional Group > alkynes > alkenes > alkene
Here the ring is cyclohexane

If we have to give numbering

Therefore, suffix $⇒$ carbonitrile
Here $⇒$ 1,2-dicarbonitrile
Therefore, Cyclohexane-1,2-dicarbonitrile

Answer: Option C. -> Propane-1,2,3-tricarbonitrile
:
C
Here, word root is prop
single bond $⇒$ ane (primary suffix)
NC- $⇒$ carbonitrile (secondary suffix)

$⇒$ 1,2,3-tricarbonitrile
Therefore,propane-1,2,3-tricarbonitrile

Answer: Option A. -> 3,6-Dimethyl-3-octene
:
A
Parent chain is
$\stackrel{1}{C}{H}_3-\stackrel{2}{C}{H}_2-\stackrel{3}{\underset{C{H}_3}{\underset{|}{C}}}=\stackrel{4}{C}H-\stackrel{5}{C}{H}_2-\stackrel{C{H}_3}{\stackrel{6|}{C}H}-\stackrel{7}{C}{H}_2-\stackrel{8}{C}{H}_3$
Word root$\Rightarrow$ oct
Double bond$\Rightarrow$suffix$\Rightarrow$ene
Two methyl groups at $C_3$and$C_5\Rightarrow$3,6-dimethyl
Therefore, 3,6-Dimethyl-3-octene

Answer: Option A. -> 2,4-Dimethylheptane
:
A
Our first step should be to choose the longest chain from the given alkane.
Although there are 9 carbonatoms in total, the longest chain contains only 7 carbon atoms.

Primary suffix =ane
Word root = hept
If we number the chain, as shown above, the two methyl substituents are at $C_4$ and $C_6$
Now, let's look at another case.

Now we have substituents at $C_{2}and{C}_4$. In both cases, there are exactly two substituents.
We choose thenumbering direction that gives the lowest set of locants.
Hence, secondary prefix = 2,4-dimethyl (since there are two methyl groups-one each at $C_{2}and{C}_4$
Hence, the IUPAC name is 2,4-Dimethylheptane