Answer is 2.
Q1. select the sid of those suppliers who supply all blue parts so we can interpret this query like this
∏sid(catalog) - ∏sid ( ∏sid(catalog x σcolor = blue(parts)) - catalog).
which gives all tuples.
Q2. 0 tuple selected because natural join with null set gives you null and second part gives you null.
Q3.Again 0 tuple selected.