The most appropriate word for blank (9) in the passage is Option B, “infants.”
The blank speaks about deformity formed in the bones. This might refer to a category in general and not to anyone in particular. Thus, the only word which describes a plural common noun/ group is the word ‘infants.’, which is Option B.
All the other options “child”, “young” and “boy” are singular nouns, which cannot be used for the above blank and hence do not match the blank.