When you change the data in a file that is hard$-$linked to three others, $q,$

a$.$ the data in the file you modified and the data in all hard$-$linked files are modified because they have different inodes

b$.$ the data in the file you modified as well as the data in all hard$-$linked files are modified because they share the same data and inode

c$.$ only the data in the file you modified is affected

d$.$ only the data in the file you modified and any hard$-$linked files in the same directory are affected