Is the verb correct?

[trainer]

What traits does an effective communicator possess?

Do I use the singular form of to do or the plural form in the sentence shown above?

[sofi]

It's wrong,you have the plural none,that's why you should use the verb do

[rewboss]

What is the subject of the sentence? Or, to put it another way, who is doing the possessing?

If you're not sure, think of what the answer to the question would be. It might start off like this: "An effective communicator possesses the following traits: ..."

So what is the subject? And is it singular or plural?

[trainer]

I say the subject is the communicator which is singular and that is why I chose the word "does." I'm getting differing opinions and starting to doubt myself. Am I correct?

[rewboss]

You are correct -- the communicator is the subject. The confusion happens because, in a question, the subject comes after the verb that agrees with it -- exactly the opposite of what happens in a normal (affirmative) sentence. People may think that "Traits possesses" is ungrammatical -- and as written, it is. But as a part of the sentence you wrote here, it's perfect grammar.