True about osteochondromatosis -
Most of the guides have wrongly explained this question because they have explained synovial chondromatosis in this, question.                                                                         •
First, let me tell you that osteochondromatosis and synovial chondromatosis (synovial- osteochondromatosis) are different diseases.
Osteochondromatosis is the presence of multiple osteochondroma in the body, e.g. in multiple hereditary exostosis.
Osteochondroma is a benign tumour and occurs in teenager and young adults.
So, the answer to this question is b, i.e. manifest at adult.
If the question would have been synovial osteochondromatosis then the answer is b & c.
Malignant
Manifest at adult
Knee is the most common size
Common in women