The nurse is assessing the symmetry of the client's chest. The nurse recognizes that a client's symmetric expansion of the chest is best confirmed by:

A. Nystagmus in extreme superior gaze: Nystagmus is an involuntary eye movement and is not a normal finding, especially in extreme superior gaze. Nystagmus can be indicative of neurological issues and requires further evaluation.

B. Slight amount of lid lag when moving the eyes from a superior to an inferior position: Lid lag refers to a delay in the downward movement of the upper eyelid during eye movement. This can be a sign of hyperthyroidism and is not a normal finding.

C. Parallel movement of both eyes: This is the correct answer. During the diagnostic positions test, the nurse should observe parallel movement of both eyes in all directions, indicating normal extraocular muscle function and coordination.

D. Convergence of the eyes: Convergence refers to the inward movement of both eyes when focusing on a close object. While convergence is a normal phenomenon, it is not specifically assessed during the diagnostic positions test, which primarily evaluates the range of motion and coordination of the extraocular muscles.

A. Percussion of the posterior chest: Percussion helps assess the underlying structures of the chest but does not directly confirm symmetric chest expansion.

B. Inspection of the shape and configuration of the chest wall: Inspection is a crucial part of assessing chest symmetry. Any deformities, asymmetry, or abnormalities in the shape and configuration of the chest wall can be visually identified.

C. Placing the palmar surface of the fingers of one hand against the chest and having the client repeat "ninety-nine": This technique, known as tactile fremitus, involves feeling for vibrations or tremors while the client repeats a phrase. While it can provide information about underlying lung conditions, it's not primarily used to confirm symmetric chest expansion.

D. Placing hands sideways on the posterolateral chest wall with thumbs pointing together at the level of T9 or T10: This technique, known as chest expansion measurement, is used to assess symmetric chest expansion. Placing hands in this manner allows the nurse to feel for bilateral chest expansion during inspiration, ensuring that both sides of the chest expand symmetrically.