Bacterial flagella have a very complex structure composed of 42 distinct proteins. what is the most likely explanation for the evolution of these complex structures?

The answer is 'Exaptation'. This term describes the change in the function of a trait developed during evolution, i.e., a trait that evolved to fulfil one function, eventually serves to fulfil another function for which it was not originally evolved. It is thought that this is common in animal anatomy and behaviour. For example, bird feathers may have initially evolved for temperature regulation, but later were adapted for flight. 

The correct answer to this question would be c