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
The answer to this question is C. miran
