¿Hay aquí médico?

The sentence is a question asking if there is a doctor here. The options are 'nada' (nothing), 'algunas' (some - feminine plural), 'nadie' (nobody), and 'algún' (some - masculine singular). Since 'médico' is masculine singular, 'algún' is the correct word to complete the sentence, forming '¿Hay aquí algún médico?' (Is there a doctor here?). The correct answer is algún.