The incorrect verb is "believe" and the correct form of it is "believes"
Step-by-step explanation:
In sentences, a singular subject should be followed by a singular verb; otherwise, there is a subject-verb agreement error because the subject does not agree in number (singular in plural) with the verb.
This error occurs in the sentence presented because the subject "the group" is singular and this is followed by the present tense verb "believe", which is used with plural subjects such as we or they. This means the correct verb form is "the group rallying to protect Lake Tahoe believes" as in this way both the subject and the verb are singular.