Following stereo-structure of trataric acid represents

Following stereo-structure of tartaric acid represents :

The following structure represents

Which of the following represents structure of polyglycolic acid?

Which of the following represents structure of polyglycolic acid?

Among the structures given below which one represents nitrolic acid?

The compound shown in the below undergoes racemization on reaction with aqueous acid . Which of the following structures best represents the intermediate responsible for this process ?