The <set difference> $\varnothing\setminus\varnothing$ is the <empty set>. Its <power set> contains exactly one element, the <empty set> itself. Therefore \b[the <cardinality> is $1$], not zero.

Solution

Past exam of the year 1 of the computer science course of the University of Oxford / 2019 / Discrete mathematics, continuous mathematics, and probability / 1 / b / i / Solution

 Solution (source code)