Answer: quieren
Let’s begin by the fact that a verb in infinitive has two parts:
-The ending: ar, er, ir
-The stem, which is everything else (except the ending, of course)
For example, the Spanish verb querer (to want/to love)
The ending is er
The stem is quer
Now, some Spanish verbs change their stem in a predictable way when they are conjugated.
There are three basic different ways in which the stem of a verb can change in the present tense:
1. From e to ie
2. From o to ue
3. From e to i
Being this the first case, where the stressed letter e in the stem of the verb will change to ie in certain forms of the verb. For example, if we conjugate the verb querer in present with the different personal pronouns we have:
1st person singular Yo: quiero
2nd person singular (informal) Tú: quieres
2nd person singular (formal) Usted: quiere
3rd person singular El/Ella/eso/esa: quiere
1st person plural Nosotros: queremos
2nd person plural Ustedes (In latinAmerica): quieren
2nd person plural Vosotros (In Spain): queréis
3rd person plural Ellos/Ellas: quieren
As you can read from the list above, the only verb forms that did not change the stem is queremos.
Now, from the options given in this question and in accordance with the explanation given, the verb form that has changed its stem is:
quieren