Choose the option that would fill in the blanks meaningfully in the sentence(s) below:

______ the importance of ‘horizontal stratification’ ______ higher education is widely acknowledged, ______ attention has been applied to horizontal stratification ______ compulsory schooling.

From an initial reading, we can understand that the two parts of the sentence are contrasting each other. So the perfect fir for the first blank is "while".
Since in the first case the importance is widely acknowledged, in the second case it has to be the opposite, so the best fit for the third blank would be "far less".
In the second and fourth blank, we need an apt preposition, and the correct one would be "within".

So the correct option is B.