I'm pretty sure you would put "Leo" in the blank space because the verb "leer" means to read and the sentence prior to the blank space says "the magazine 'Cosas (or Things)' is my favorite. I read it every month. Hope this helps!
un - masculine singular
una -feminine singular
unos - masculine plural
unas - feminine plural
UN lápiz - a pencil
UNAS tareas - some homework
UNOS borradores - some erasers
UNAS mochilas - some backpacks
UNOS libros - some books
UNA calculadora - a calculator
UNOS papeles - some papers
UN examen - a test
Many times we can find the right article by paying attention to the noun ending:
unAS tareAS
unOS librOS
La pagina web = la pagina social
