The correct option is A 16
The set is defined by:
Y={x:x≤16 & x∈N}, which can be interpreted as the set of all natural numbers x such that x≤16.
So, in Roster form we can write it as:
Y={1,2,3,4,...,14,15,16}. As the number of distinct elements in the set is 16.
Hence, it's cardinality =16.