What is the structure of the third member of the homologous series of aliphatic aldehydes?

Aldehydes are isomeric with:

Which of the following aldehydes has a buttery odour?

How many total ketones are possible for the molecular formula $C_5H_{10}O$?

With reference to the carbonyl group,which statement$(s)$ is/are correct?
$A$. Carbonyl carbon is electrophilic
$B$. Carbonyl carbon is nucleophilic
$C$. Carbonyl oxygen is electrophilic
$D$. Carbonyl oxygen is nucleophilic

Carbonyl compounds are usually