Please Don't judge a question from any ISRO papers,
ISRO asked completely wrong question too , they took questions either from Timothy Williams book or Old gate papers..
yes this question when asked in GATE 2007 that time no official answer key , also GATE committee followed old convention that time. ISRO took this as a reference.
This year ISRO even asked completely wrong question too
Just read all discussion here https://gateoverflow.in/128643/isro2017-26
finally ISRO admit that this question was wrong in their newly released answer key. They said no options are correct.
In GATE if this concept comes again in next year, we should follow New convention , as new convention is based on RFC 1878 while old convention is based on RFC 950 , so new one is more acceptable .
And yeah read what Wikipedia says "
The RFC 950 specification recommended reserving the subnet values consisting of all zeros and all ones (broadcast), reducing the number of available subnets by two.
However, due to the inefficiencies introduced by this convention it was abandoned for use on the public Internet, and is only relevant when dealing with legacy equipment that does not implement CIDR. The only reason not to use the all-zeroes subnet is that it is ambiguous when the prefix length is not available. RFC 950 itself did not make the use of the zero subnet illegal; it was however considered best practice by engineers."
and GATE committee nowhere mention which convention to follow ..
If GATE committee don't give marks for new convention we can challenge them !