Right answer:
A) cambias
Cambias is the conjugation of the verb cambiar that matches the second person singular tú (informal you) in the simple present. So we use the simple present to talk about either habitual actions or routines. The other options are cambiáis (that matches vosotros), cambia (that matches él, ella or usted) and cambian (that matches ellos or ustedes). Finally:
Tú cambias los pantalones negros por los pantalones blancos.
