Which verb form has changed its stem?
queréis
quieren
queremos
querer

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: