Relative pronoun agreement

Question

Please, could someone explain why the following is given as a correct option? I understand that relative pronouns agree in gender and number: Les filles à qui je pense sont géniales.

Maarten Kamp · Accepted Answer

Kalpana, 
 ‘ qui ‘ is invariable, and constructs of ‘ preposition-qui ‘ as in ‘ à qui ‘ in this lesson, also remain invariable.
It is the expressions that derive from the variable ‘ quel ‘ that are also variable and have agreement even when in contracted form with prepositions, as in auquel/á laquelle/auxquels/auxquelles in this lesson. 
It could and should  be made clearer in the lesson that the note in the following only applies to the latter ( quel) forms, as it is not completely clear as currently written : 
“ There are two ways to introduce relative clauses with the preposition à:
- à qui (about whom / to whom), which can only apply to living things
- The contracted forms of à + le / la / l' / les + quel: auquel / à laquelle / auxquels / auxquelles (which)
- Note that these forms agree in gender and number with the object they refer to.