@Kunal Kadian
what is inverse element if it is group?
@srestha
For any element a, inverse b = (m-a) mod m
@Nandkishor3939 I think idenitity element should belong to the given set, m is not in given set. Moreover identity element, if exists, is unique. here it is $0$
@srestha Yes, If you take m= 5, then inverse of 1 is 4.
For any general case, inverse of 1 = m-1