SAT Question c5d39bc7: Form Structure And Sense answer and explanation

English - Form Structure And Sense - Pronoun Antecedent Agreement
0:00
FREE OFFICIAL BANK
Scientists believe that, unlike most other species of barnacle, turtle barnacles (Chelonibia testudinari) can dissolve the cement-like secretions they use to attach ______ to a sea turtle shell, enabling the barnacles to move short distances across the shell’s surface.
154
Difficulty:Easy
Which choice completes the text so that it conforms to the conventions of Standard English?