There is no need to use Union operator here.
Just because they say you can use operators from $(∪, −)$ we don't need to use both of them.
Also they are saying that only the minimum number of operators from (∪, −) which is equivalent to $R ∩ S$.
My expression is Minimal.
air1ankit see this example
Can you please say Is it necessary to use both U,-?
Can I represent R ∩ S=R-(U-S) ?
where U is the universal set.
Question says only the minimum number of operators from (∪, −) which is equivalent to R ∩ S. Using union is unnecessary here !