Which medication is used to alleviate allergy symptoms and is a nasal corticosteroid?

Fluticasone is the correct choice because it is a nasal corticosteroid specifically used to alleviate allergy symptoms such as nasal congestion, sneezing, and runny nose. Nasal corticosteroids work by reducing inflammation in the nasal passages, making them effective for managing symptoms associated with allergic rhinitis. Fluticasone is often available as a nasal spray, allowing for direct delivery to the site of action in the nasal mucosa, leading to rapid relief from allergy symptoms.

The other medications listed serve different purposes. Fexofenadine is an antihistamine that counteracts the effects of histamines in the body but is not a corticosteroid. Diphenhydramine is also an antihistamine that can help relieve allergy symptoms but has sedative effects and is not a nasal corticosteroid. Salmeterol is a long-acting beta-agonist, primarily used for asthma or chronic obstructive pulmonary disease (COPD), and does not have a role in managing allergy symptoms directly. Thus, fluticasone stands out as the appropriate choice for a nasal corticosteroid to alleviate allergy symptoms.