Options start with $125.126.11$ but the figure has $135.126.11$ so might be a misprint.
Other than that,
We could do the following (125.126.11.000111 as mask) BUT
125.126.11.31 = 125.126.11.00011111
125.126.11.28 = 125.126.11.00011100
All $0’s$ and all $1’s$ are reserved for broadcast and network name and here we have $11$ and $00$ in the host id.
So we cannot use the above mask as host ID of A and B clash with broadcast and network id respectively.
So we have to do (125.126.11.00 as mask )
125.126.11.31 = 125.126.11.00011111
125.126.11.28 = 125.126.11.00011100
This is the longest subnet mask for which both the host id’s don’t have all $0’s$ or all $1’s$.
So, Subnet 1’s mask that has maximal network prefix length can be, $125.126.11.0/26$
