the guard standing in the open door held the flats in his hands make it complex

Dear Student, 

I am assuming that the statement is : 

The guard standing in the open door held the flags in his hands. ( flats does not make any sense)

Accordingly, here is the answer:

The guard, who was standing in the open door, held the flags in his hands. 

If this was not what you had in mind, please re-post the clear and complete query in a separate thread. Our experts will be happy to help you there. 

Regards,