Une is used when the noun is a féminin.
Example
une gomme — an erasure. gomme is a féminin noun so we use une instead. (most words that have 'e' as the last are most likely a féminin
une table
Example of 'des'Des is used when the noun is plural. Similar to English where you add 's' or 'es', it becomes plural. For French, most words that add 's' or 'x' are most likely plural.
Example
des ciseaux
des enfants
Example of 'un'Un and Une both have same meaning. It means the article 'a' in English. But things in French have genders so both are separated for gender uses. Un is for masculin.
Example
un homme
un livre
From the sentences, our noun is ordinateurs (computers). The word is a plural because we have 's' at the last of word. Therefore, we cannot use une or un.
Answerb) des