I worked like this
If I see, return value t is t+p.
According to last definition of t, t=p*q=6*7.
So, t=p*q+p and return this t.
and I can directly evaluate that expression using 2 registers only.
So minimum register needed is 2.And infact, the program effect is same, even if I skip computing u,r,s.
But the correct answer is given 5.
I want to know if my approach is correct?