The stem of a verb is found by.. A removing the -ar, -er, or -ir from the infinitive
B using the first three letters of the infinitive
C adding the correct ending to the infinitive based on the subject
D using the infinitive form

A) removing the -ar, -er, or -ir from the infinitive

Explanation:

The stem of a verb is what remains when you remove the infinitive ending (‐ar, ‐er or ‐ir).

In irregular verbs in which the stem changes, these verbs are called stem‐changing verbs. You must learn each particular verb and remember that it is a stem‐changing verb.