the option b is correct . there is just a small definition of multivalue dependencies which . if a->> c is a dependency it means for a ,c has more than one value.
here it is saying whenever a appears b has same value which is true in case of functional dependency . if a->b exist for every a if repeats there should be same b . so a-.b exist here and it is clearly saying c is not same which means for same a it contains multipule value of c. so answer is b .
why not c ??
just consider the case
a is 1
and for c
now u can see that for a we have multipule value of c but c does not have multiple values for a . so we cannot say c->>a . but we can say a->>c. and question is saying c can take different value not a .