Which of the following letter cluster will replace the blanks in the given series?

ca_pcaa_caa_caap_aap

aapp

In the series, 

ca_pcaa_caa_caap_aap

caap is the repeated word:

ca_p | caa_ | caa_ | caap | _aap

The complete series is :

ca_p | caa_ | caa_ | caap | _aap

   a          p        p              c       

The letter cluster required is appc