Tricky sentence construction! The problem phrase in your sentence is who turn out one best seller after another. This phrase is referring to those gifted writers, which is a plural subject. Therefore, the use of turn, the plural form of the verb, is correct.
The problem phrase is called a restrictive appositive. An apposition is a construction that uses two noun phrases next to each other with one phrase adding information about the other. Your sentence tells us that she is a gifted writer; it tells us that she belongs to a group of writers who author repeated best sellers. As a restrictive appositive, the problem phrase is critical to the meaning of the sentence and is not separated from the other phrase with a comma.
My understanding is that deterimining whether or not the group is singular or plural relies on the actions of the group's members. (This is also how I teach it to my 11th and 12th grade students.) If the members of the group are all acting as one, a singluar verb is required. For example: "The orchestra IS meeting in Room 220." If, however, the group members are doing a number of different things, a plural verb form is required: "The orchestra ARE meeting in Rooms 102 and 220." We hear it on the news all the time with jury decisions. "The jury WAS unanimous in the decision to convict Mr. Smith." When they disagree, though, the sentence changes to "The jury WERE in two different camps when it came to a verdict. Three believed Mr. Smith was guilty while nine felt he was innocent."
While answer number one is totally correct, I don't think 'turns' is wrong.
In your example 'turn' refers to the group of writers (of which she is one) i.e. There are several members of the group, so they are plural. 'They turn out bestsellers'.
But it is also ONE group. So, 'The group turns out bestsellers.'
Look at this.
- The government is trying hard.
- The government are trying hard.
- UPS is a courier service.
- UPS are a courier service.
- The Chicago Bulls are a group of people.
- The Chicago Bulls is a group of people.
Is a group of 10 people singular or plural.... ? I don't think there is a final answer to that question.