Subject-verb agreement refers to the subject of the sentence (in this case, the activist) 'agreeing' with the verb that this subject is performing, so that the sentence is grammatically correct and makes sense.
the subject is the activist (the noun) and the verb is 'has'.
Since there is only one activist this is a singular noun, and so u hav to use the singular form of the verb, 'has'. If there were two or more activists the subject would be plural and so u would hav to make the verb plural, as in 'they have criticized'.
PS check spelling: "horseracing" :)
hope this helps