Which of the following is a perfect square?

The best way would be to see the range in tens and solve after looking at the units digit
lets see all choices--
A. 649
20^2=400 and 30^2=900..
since only 3 and 7 have the perfect square ending in 9, we require to try only for 27 as it is more than 25^2, 625
it is not 27^2, as next square of 26 itself after 625 will be >625+2*25,675
actually 26^2 will be 25^2+25+26
B. 961
between 30^2 and 40^2...
only 1 and 9 have squares ending with one..
since it is very close to 30^2, 900.. it can be 31^2
CORRECT
C. 1664
between 40^2 and 50^2..
closer is 42, but that should be atleast 1600+41+42.. NO
D. 2509
between 50^2 and 60^2..
very close to 50^2 so next square of 51 will be at least 2500+51.. no
E. 100000
odd number of 0s, not possible
ANSWER:B