In the above image can we say that y5 is the Co-domain ? Why or Why not ?

In the given relation, X is domain and Y is codomain. So y5 is also part of codomain with reference to the definition of co-domain

Co-domain: It is set of values which may possibly come out of a function for a given set of elements in domain. So y5 is part of co-domain set.

Above mapping is a function and it is Y is a function of X where as reverse is not true because for a fixed value in set Y we have two values in set X which violates definition of function