The answer is un
Explanation: the sentence states that the director wants to buy a school bus. Since she only wants one school bus you can remove the options with s on them (unas, unos) since they stand for more than one. Then, to choose between “una” or “un” you have to determine if a school bus is a masculine or a feminine term. In this case, the term is masculine and therefore uses “un” instead of “una” because “una” only describes feminine terms such as “una carta” (a letter).
Hope this helps.