Which of the following subnet masks would allow for 192.168.0.3 and 192.168.0.240 to be in the same subnet?

With a 255.255.255.0 mask, the network address is 192.168.0.0 and the broadcast is 192.168.0.255, so all addresses from .1 to .254 including .3 and .240 fall within the same subnet. Any mask with a longer prefix splits the address space into smaller blocks that separate .3 (low range) from .240 (high range).

A 255.255.255.128 (/25) mask divides the space into 192.168.0.0-127 and 192.168.0.128-255, placing .3 and .240 in separate subnets.

A 255.255.255.192 (/26) mask creates four /26 blocks - .3 falls in the 0-63 block and .240 falls in the 192-255 block, so they are in different subnets.

A 255.255.255.248 (/29) mask creates very small 8-host blocks - .3 is in the 0-7 block and .240 is in the 240-247 block, placing them in entirely different subnets.