Which of the following aldehydes has a buttery odour?

Formalin is an aqueous solution of

Which is the main functional group in Acrolein?

Write the structures of the following compounds:
$(i)$ $\alpha$-Methoxypropionaldehyde
$(ii)$ $3$-Hydroxybutanal
$(iii)$ $2$-Hydroxycyclopentanecarbaldehyde
$(iv)$ $4$-Oxopentanal
$(v)$ Di-sec-butyl ketone
$(vi)$ $4$-Fluoroacetophenone

Butanal is an example of

Which of the following is a simple (symmetrical) ketone?