The right answer is option C.We need to know what the modifier presumed to be genetic or partially genetic in origin is intended to modify. It is intended to modify
a severe disorder.
A.
Presumed to be genetic or partially genetic in origin, about three in every 100 children are born with a severe disorder. Incorrect. This is because the modifier is misplaced. The modifier will make more sense if it comes after
about three in every 100 children are born with a severe disorder.
B.
Born with a severe disorder presumed to be genetic or partially genetic in origin, are about three in every 100 children. This is incorrect. The participial phrase
Born with a severe disorder presumed to be genetic or partially genetic in origin cannot be the subject of the verb are. This sentence is incorrect. At best we expect a plural noun phrase or a noun as the subject of the verb
are.
C. About three in every 100 children are born with
a severe disorder presumed to be genetic or partially genetic in origin. Correct. The modification is correct in this option and the meaning is very clear.
D. About three in every 100 children,
presumed to be genetic or partially genetic in origin, are born with a severe disorder. wrong. We cannot presume three out of every 100 children to be generic or partially genetic even before they are born. Presumed to be genetic or partially genetic in origin is expected to modify genetic disorder.
E. Severe disorders
that are presumed to be genetic or partially genetic in origin
are found in about three in every 100 children
who are born. Option E is wordy and not concise when compared with option C. The modification is correct in this option, as presumed to be genetic or partially genetic in origin correctly modifies severe disorders.