What is the primary function of the vagus nerve?

The primary function of the vagus nerve is indeed the regulation of heart and lungs, making this option the most accurate. The vagus nerve is the longest cranial nerve and is a critical component of the autonomic nervous system. It helps control involuntary functions such as heart rate, respiratory rate, and gastrointestinal activity. Through its extensive network, the vagus nerve sends signals that can influence the heart's pacemaker and help slow down heart rate, as well as modulate the activity of the lungs, aiding in the regulation of breathing.

The other options represent functions associated with different cranial nerves or neural pathways. Motor control of the abdomen is primarily handled by other nerves, while sensory information from the face is linked to the trigeminal nerve. Control of eye movement falls under the function of the oculomotor, trochlear, and abducens nerves. Hence, these other choices do not capture the unique role of the vagus nerve in maintaining cardiovascular and respiratory function.