I presume it was actually "rappelles" not 'rapelles' given as the correct answer ? 

It is just the normal conjugation of verbs with 'tu' - apart from a few exceptions that end in 'x' (eg veux, peux), and in the dropping of the 's' in the imperative conjugation of '-er', '-frir' and '-vrir' verbs (unless followed by 'en' or 'y', when the 's' is retained) - all simple tense conjugations of verbs with 'tu' end in 's'. Nothing to do with plural, as 'tu' is always singular.