Which of the following test is likely to helpful in diagnosing a patient presenting with an itchy annular plaque on the face?

Patient in the question is most likely a case of fungal infection of the skin of the face. So the best method to diagnose the condition would be a KOH mount, in which the keratin of the skin will be lysed and the fungal hyphae and other elements are clearly visualised. KOH mount in a case of Tinea corporis will show "Spaghetti and meatballs" appearance. Ref: Laboratory Diagnosis of Infectious Diseases: Essentials of Diagnostic Microbiology By Paul G. Engelkirk, Page 491; Combating Fungal Infections: Problems and Remedy By Iqbal Ahmad, Pages 179-80.