Right answer:
habrán cambiado
We use the future perfect when talking about something that will have been completed at a certain point in the future. Moreover, we can also use this tense to point out probability. In addition, this tense is often used to refer to what might have or could have happened. The formula of this tense is:
Simple future of haber + past participle
In this way habrán is the conjugation of the verb haber for the third person plural in the future perfect and cambiado is the past participle of the verb cambiar. So:
En el futuro, los teléfonos quizás habrán cambiado a ser más pequeños
