The Answers Are: prepares (a), preparing (c), prepared (a)
After ‘always’, a simple tense verb, we can have a past tense or a present tense verb. If we look at the end of the sentence, we see ‘we always have a wonderful time’, meaning this is the present, not the past–so we need a present tense verb after always–so the answer to the first option is ‘prepares’.
After ‘by’ we need a gerund, because by introduces nouns of process or conveyance and actual things; think of how we say ‘go by train’, ‘solve the problem by using calculus’, the cat reached the window by jumping.
After ‘is always’ we need an adjective form or a continuous verb form, because ‘is’, a linking verb, and ‘always’ as an adverb require a description or a habitual action, but the choices offer only one ‘ing’ option and it is needed elsewhere!