Dear Student,
The step by step reactions are as follows.
As given on oxidation of A with dichromate it gives acetic acid and malonic ester that gives the information of the presence of an acid and ether group in A and as molecular formula of A is C5H8O3.
The reaction is as follows :
(i) So, A will be .
Now on heating A with sodalime it undergo decarboxylation to give B.
The reaction is as follows :
(ii) So, B will be .
Now B reacts with HCN undergo nucleophilic addition reaction over aldehyde .
The reaction is as follows :
(iii) So, C will be ​​
Now, C reacts with Thionyl chloride to give D : Nucleophilic susbstitution :
The reaction is as follows:
(iv) So, D will be
Now , D reacts with KCN to give E : Nucleophilic substitution :
The reaction is as follows:
(v) So, E will be
Now, E on alkaline hydrolysis gives F :
The reaction is as follows:
(vi) So, F will be ​
Regards