Subject-verb agreement is basically an agreement between the subject and the verb, in a sentence. "The subject must agree with the verb in number (singular or plural)." If the subject is singular, then the verb must be singular, for example: The dog is barking. (dog is singular, 'is' is a singular verb)
When the subject is plural, the verb also must be plural, for example: The dogs are barking. (dogs are plural, 'are' is a plural verb)
The subject in the given sentence is "everybody", (the subject is that thing or person that performs the action of the verb). 'Everybody' is a pronoun that refers to every body, or every person. It is singular, hence the verb that follows will also be singular.
From the options, 'is' is a singular verb whereas 'are' is a plural verb. (Consider the given examples.) The singular verb 'is' agrees with the singular subject 'everybody'. Hence option B is correct and A is incorrect.
When 'has' is used as the main verb, it means 'to possess or own something'. When placed in the given sentence, it does not express a complete thought: Hence option C is incorrect.
'Have' is the plural form of 'has'; neither does it agree with the subject, nor is it relevant in meaning here (the same as 'has'). Hence option D is incorrect.