Answer:
Los lápices -> El lápiz (it can look like it's irregular but it's not, in Spanish you write "c" before e and i to make the sound "th" but "z" before a, o and u to make that sound).
El cuaderno -> Los cuadernos
Una regla -> Unas reglas
Unos papeles -> Un papel
Los borradores -> El borrador
La calculadora -> Las calculadoras
Un amigo -> Unos amigos
Las niñas -> La niña
Step-by-step explanation:
Native speaker over here. I hope that is helpful.