20 !

a gene with the sequence attcattca underwent duplication after a few generations. which sequence represents this gene after the mutation?

a. attcattca
b. acttactta
c. attcacattca
d. atccattca
e. atcattca

All you have to see whether or not a duplication mutation has occurred, is simply count the number of nucleotides present in the original DNA strand and compare that number to the nucleotides present in the options. The only sequence which has substantially more nucleotides would be C. Thus we know that certain portion had been duplicated if the sequence after the mutation is in fact C.