Which graph could represent a constant blalance in a bank account?

The third graph ( i.e. a horizontal line  ) represents  a constant balance in a bank account.

Constant balance in a bank amount means that the amount of balance in the account is same over different time period.

i.e. it did not increase or decrease over a period of time.

Hence, the graph of such a situation is a horizontal line parallel to the axis labeled as Time period.