@MartinSleziak , Sorry for late reply, I was a bit busy. The problem is of competitive exam GATE old papers. The solution is given deferent-2 in market solution books. The site for GATE old papers( gateoverflow.in/previous-years ) made by students, and explanation given there by student. I've also written some answers there. For this question there is no solution available.

@MartinSleziak , Genuinely, I'm not prepared double integral. So, I'm confused for this problem. The GATE is standard exam, but GATE's question has/have some ambiguity ( as, I realized. ). On the my post, I agreed with given comment by @stef.