To "intend" is to mean to do something. In this context, it could also be replaced with the word "want", as in, "If you want to visit your grandmother, you..". The other words do not make sense with their meaning or tense in the context of the sentence. To "assert" is to force one's dominance over something, to "contend" means to be in competition for, and to "retort" is to reply in a sharp or angry tone. The only word which fits the meaning of wanting to do something, in the future, is "intend".
