First of all , we have to understand that it is a classless addressing (CIDR notation).So here the number of addresses in a network is determined by the mask which is used .In this case the mask used is '/16' , hence,
No of network bits = 16
And no of host bits = 16
So no of hosts fror each network = 216
Now given no of subnets in each network = 1024.
Just to add the 10 bits used for subnet is taken from host bits in order to divide the network into subnets and hence will be used for defining subnet number.
So no of bits masked in total due to subnetting = 16+10 = 26
Now for subnet no. we have 10 bits and hence remaining 6 for host in a subnet.
Now we are asked the first address of 1024th subnet.For that
The 1st 10 bits = 11111111 11 (for subnet no)
last 6 bits(for a given subnet which is 1024th subnet) = 000000 (since it will be the 1st address of subnet)
So on combining we get the last 16 bits as : 11111111 11000000
The first 2 octet will remain the same since the subnet will belong to the original network only
So in dotted notation , the first address = 190.76.255.192/26 ["/26" bcoz now 26 bits are masked in total as mentioned earlier]
For last address , we set the last 6 bits to 1 as well
So in dotted notation , the last address = 190.76.255.255/26
Hence C) is the correct option.