<h2>Right answer:</h2><h3>a) Juego al tenis hace un mes</h3>
We can translate the sentence <em>I have been playing tennis for a month </em>into <em>Juego al tenis hace un mes. </em>So the group of words hace un mes comes from a common formula in Spanish:
<em>Hace + number + días/meses/años</em>
that is used to tell you something form the past until now. Since the number is uno (one), <em>días/meses/años </em><em>(plural form)</em><em> </em>changes to <em>día/mes/año </em><em>(singular form)</em>