Answer:
a. Serbia and service
Step-by-step explanation:
Guide words are terms usually found on the top of a document arranged alphabetically (example, dictionary), so as to indicate the first and last words on the page. From the alphabetical arrangement of the word "serpent", it is most likely that "Serbia" will come before "Serpent", while "Service" will be found after "Serpent".
B comes before P, and P comes before V. So, "Serbia" will be found at the top of the page. "Serpent" will be found at the middle while "Service" comes last.