One of the most famous prehistoric monuments in Europe is the mysterious site known as Stonehenge. It is located in southwestern England on Salisbury Plain, and its earliest structures dates back approximately 4,500 years to the Neolithic Age. A variety of researchers over the years has determined the evolutionary sequence of the various structures from geological and archaeological evidence. In addition, more recent research in the areas surrounding Stonehenge have revealed evidence of a Neolithic village and several gravesites, which have yielded useful information. In 2002, the tomb and remains of a man dubbed the Amesbury Archer was unearthed. Chemical analyses of his teeth have demonstrated that he grew up somewhere in the Alps and migrated to southwestern Britain. The Archer was buried with artifacts, including gold hair ornaments and various tools that indicates he was wealthy. The idea of his migrating so far has intrigued those who are exploring in areas around Stonehenge.
Which option below would be an acceptable revision of the underlined selection?
a. have determined
b. determine
c. is determining
d. No revision is necessary

The use of the verb "has" or "have" depends on the person and the number. Has is used for third-person singular whereas "have" is used for first-person and plural nouns or pronouns. And the main point to be used in this case is that "has" is for singular nouns whereas "have" is to be used for plural nouns.

Looking at the given sentence, "A variety of researchers over the years has determined the evolutionary sequence . . ." the noun "researchers" is plural. This determines the form of the verb to be used alongside it.

Thus, the acceptable revision of the verb will be "have determined".