Answer: Option D. -> 8
:
D
8 isomers will be possible.
Degree of unsaturation = 2
This means we can have a triple bond, 2 double bonds, one double bond and a ring, or 2 rings.

Answer: Option B. -> Positional isomers
:
B
Positional isomersare constitutionalisomersthat have the same carbon skeleton and the same functional groups but differ from each other in the location of the functional groups on or in the carbonchain.
The two isomers differ in the position of the double bond. So they are positional isomers.

Answer: Option A. -> 2-butene
:
A

Answer: Option D. -> 6
:
D

For 2-butene, we can further have cis-trans isomerism.

Answer: Option B. -> (CH3)3C−OH
:
B
Diethylether has a formula of $C_4{H}_{1}0O$
If you count the atoms in the options, you will get this only in$(C{H}_3{)}_{3}C-OH$

Answer: Option D. -> None
:
D
They are not isomers as their molecular formula is different.
Molecular formula of acetaldehyde is $C_2{H}_{4}O$ and of acetoneis $C_3{H}_{6}O$

Answer: Option A. -> 1-phenyl-2-butene
:
A
Only 1-phenyl-2-butene will exibit cis-trans isomerism

Answer: Option B. -> Functional isomerism
:
B
$C{H}_3-C\equiv C-C{H}_3$ is 2-butyne and $C{H}_2=CH-CH=C{H}_2$ is 1,3-butadiene.
As their molecular formula is the same but functional group is different they are functional isomers.
2-butyne has a triple bond while 1,3-butadiene has two double bonds.

Answer: Option A. -> 2-butene
:
A
The compounds with each doubly bonded carbon attached to two different groups (like Cab=Cab, Cab=Ccd) exhibit geometrical isomerism i.e., cis and trans forms. The geometrical isomerism arises due to the restricted rotation of the double bond.
However, even though there is restricted rotation for a triple bond, alkynes do not exhibit geometrical isomerism, since the triply bonded carbons are attached to only one group each.

Answer: Option D. -> RCH2NO2
:
D
Nitro compound exhibit tautomerism $RC{H}_{2}N{O}_2$