Fill in the blank with the plural form of el pupitre: . what is the plural form of el pupitre? ?

El pupitre in Spanish means desk.
As you can see by the article 'el,' the noun pupitre is masculine in Spanish. This means that if you want to make a plural form of that article, it would be los for masculine gender, or las for feminine gender - but here we need masculine.
Also, in Spanish, when you want to create a plural form of a word, you simply as the suffix -s at the end. So pupitre becomes pupitres in plural.
Thus, the correct answer is los pupitres. 

The answer is they