For TC:
abracadabra
abrabracada

why don't the prefixes are:
abra abracada a?

and also

why for TC:
abracadabra
arbadacarba

the prefix is not:
a?

why don't the prefixes are:
abra abracada a?

You can write this prefixes, this is the correct answer.

why for TC:
abracadabra
arbadacarba

the prefix is not:
a?

Because the word must consist entirely of prefixes.