Murder of a new-born child

'Infanticide' means 'the killing of an infant'. An 'infant' is 'a very young child or new-born baby'.
This word means the same as the given expression. Hence it can be substituted for and option 4 is correct.
'Homicide' is 'the killing of one person by another'.
'Regicide' means 'the action of killing a king'.
'Fratricide' means 'killing one's brother or sister'.
These words do not mean the same as the given phrase and hence cannot substitute for it.