Remember what division means in basic operations –
When $S$ is empty $(\pi_A(R)$ x $S)$ is also empty. (The cartesian product of any non-empty with an empty set gives an empty set)
Further $\pi_A(\pi_A(R)$ x $S$ – $R)$ is also empty. (Subtracting from an empty set also yields an empty set).
Thus $R/S$ when $S$ is empty is $R$ itself. So, all $4$ tuples (SID, CID) from $Enrolled$ relation should be present in the result query.
Learn the literal meaning of division operator and how we come up with the above relational algebra expression for that operator – https://youtu.be/Zzjy-q667r0
