"A" is the correct choice :)
Try to avoid distractions and really pay attention to what you are doing
Bluebirds should have a round entrance hole measuring 1½" to 19/16" in diameter, or a 1⅜" x 2¼" vertical oval hole, or a 1⅛" to 13/16" horizontal slot entrance.
Was is not an action, so it's a linking verb.