(a) CH3−CH2−

OH ||

C

−CH2−CH3 and CH3−CH2−CH2−

OH ||

C

−CH3 are structural isomers.
Structural isomerism arises when two compounds have the same molecular formula but differ in structure i.e., arrangement of atoms.
(b)

D |

C

| H

=

H |

C

| D

and

D |

C

| H

=

D |

C

| H

are geometrical isomers.
(c) H−

+ ‌ OH ||

C

−OH and H−

OH |

C+

−OH are resonance stabilized structures. Such structures arise when the arrangement of atoms remains the same but the bonding pattern differs.