The commonest type of cataract is –
You must be thinking, why not a blue dot?
Carefully read the question. Here the question is about the commonest type of cataract (not congenital cataract).
A most common type of cataract is senile cataract (Age-related cataract).
A most common type of senile cataract is cuneiform cataract (70%).

"In general, the predominant form can be given as cuneiform (70%), nuclear (25%), and cupuliform (5%)". — Anatomy & Physiology of eye
"Cortical senile (Cuneiform) cataract is the most common form of senile cataract prevalent in aging population".
Blue dot
Zonular
Cupuliform
Cuneiform