The refractive index of medium c with respect to a is denoted as nca.
As per the definition, nca=nanc, where nc is the absolute refractive index of medium c and na is the absolute refractive index of medium a.
Now, nca=nanc=nanb×nbnc
nanb=nba and nbnc=ncb
Thus we get,
nba×ncb=nca
So the answer is ncb.