Which hybrid orbitals are used by carbon atom in the following molecules?
$(a)$ $CH_3-CH_3$
$(b)$ $CH_3-CH=CH_2$
$(c)$ $CH_3-CH_2-OH$
$(d)$ $CH_3-CHO$
$(e)$ $CH_3COOH$

(N/A) The hybridization of a carbon atom depends on the number of sigma bonds and lone pairs attached to it.
$(a)$ $CH_3-CH_3$: Both carbons are bonded to $4$ atoms ($3$ $H$ and $1$ $C$),so both are $sp^3$ hybridized.
$(b)$ $CH_3-CH=CH_2$: The terminal $CH_2$ carbon is bonded to $3$ atoms ($2$ $H$ and $1$ $C$) and has a double bond,so it is $sp^2$ hybridized. The central $CH$ carbon is also bonded to $3$ atoms ($1$ $H$ and $2$ $C$) and has a double bond,so it is $sp^2$ hybridized. The $CH_3$ carbon is bonded to $4$ atoms,so it is $sp^3$ hybridized.
$(c)$ $CH_3-CH_2-OH$: Both carbons are bonded to $4$ atoms,so both are $sp^3$ hybridized.
$(d)$ $CH_3-CHO$: The carbonyl carbon $(C=O)$ is bonded to $3$ atoms ($1$ $C$,$1$ $O$,$1$ $H$),so it is $sp^2$ hybridized. The $CH_3$ carbon is bonded to $4$ atoms,so it is $sp^3$ hybridized.
$(e)$ $CH_3COOH$: The carboxyl carbon $(C=O)$ is bonded to $3$ atoms ($1$ $C$,$1$ $O$,$1$ $O$),so it is $sp^2$ hybridized. The $CH_3$ carbon is bonded to $4$ atoms,so it is $sp^3$ hybridized.