Final answer:
The correct sentence should be "Mr Mosley is trying to concentrate on his yard work but a swarm of bees keeps distracting him," because 'swarm' is a singular subject that requires a singular verb for proper subject-verb agreement.
Step-by-step explanation:
The question you asked is regarding the correct construction of a sentence, specifically dealing with the subject-verb agreement. The sentence provided, "Mr Mosley is trying to concentrate on his yard work but a swarm of bees keep distracting him," contains an error in subject-verb agreement.
The subject of the sentence is a 'swarm of bees' which is singular, and therefore, the verb should be 'keeps' instead of 'keep' to match the singular subject. Hence, the correct sentence should be "Mr Mosley is trying to concentrate on his yard work but a swarm of bees keeps distracting him." In standard English, it's important for the subject and verb to agree in number.