Choose the best verb to complete the sentence. vengo digo dice viene charlie y sophie quince años. el maestro buenos días cuando entra en la sala de clase. yo siempre la verdad "truth". mi tía a visitar.

Let's analyze each sentence as follows:

Correct tienen

The list has a little mistake because the correct conjugation is tienen not vengo, because this sentence talks about the age of Charlie y Sophie. So tienen is the conjugation of the verb tener that matches the third person plural in the simple present. This person is ellos, so Charlie y Sophie can be replaced by ellos. Moreover, we use the verb tener to tell the age of a person. Finally:

Charlie y Sophie tienen quince años.

Correct dice

Here the subject of the sentence is el maestro and he is carrying out an action that is to say something. Therefore, we must use the verb decir. Since this sentence matches the third person singular, that is, el maestro can be replaced by él (he), then the conjugation of the verb decir that matches él is dice. Finally:

El maestro dice buenos días cuando entra en la sala de clase.

Correct digo

Yo is the subject of the sentence here and means I in English. When someone says the truth, in Spanish we use the verb decir. Since yo is the subject this sentence matches yo, then the conjugation of the verb decir that matches this person is digo. Finally:

Yo siempre digo la verdad

Correct viene

Mi tía can be replaced by ella, that is the subject pronoun for the third person singular in feminine form translating into she. Thus, viene is the conjugation of the verb venir that matches the third person singular too. In this way, we can write this sentence as follows:

Mi tía viene a visitar

Note. It's better to use a Spanish reflexive pronoun (me for example). So this sentence sounds better as mi tía me viene a visitar.