(i)

First ethanal undergoes an aldol reaction with reacting another molecule of ethanal, under basic conditions. On treatment with dilute alkali, ethanal produces 3-hydroxybutanal.

Then 3-hydroxybutanal gives butane-1, 3-diol on reduction with NaBH₄. 

The reactions are as follows:

(ii)
When ethanal undergoes an aldol condensation reaction then But-2-enal is formed as a product. In this reaction, ethanal is treated with dilute alkali and gives 3-hydroxybutanal which on heating produces but-2-enal.

The reaction is as follows:

(iii)
In the first step, ethanal undergoes an aldol condensation reaction and But-2-enal is obtained as a product. 

In the second step, the aldehyde group reacts with tollens' reagent followed by acidification.

The reactions are as follows: