Let's analyze each sentence as follows:
1. Charlie y Sophie _____ quince años.
Correct answer: 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.
2. El maestro ______ buenos días cuando entra en la sala de clase.
Correct answer: 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.
3. Yo siempre _____ la verdad
Correct answer: 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
4. Mi tía ______ a visitar.
Correct answer: 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.
