$1.$ Please identify the number of Habits a User can and must have $($min and max cardinality$).$

In the ER diagram, this has the cardinality symbol between user and habit showing that a user can have many different habits or zero. Therefore, the minimum cardinality is $0,$ and the maximum cardinality is many. Moreover, a user doesn$$t need to have any habit.

$2.$ What is the relationship between Checkin and Comment?

a$.$ One$-$to$-$one relationship

b$.$ One$-$to$-$many relationship

c$.$ Many$-$to$-$many relationship