ANS:B - modification anomalies.

Modification anomaly comes when we, say, modify an attribute value and this modification has an undesired effect on the database. Suppose we changed an Attribute Value Dep_name in department table from marketing to research and, in case, this modification leads you to change the attribute value DName to 'Marketing' in Manager table as well (Because Dep_name in Department Table was a PK and DName in Manager table was an FK).

So, as a solution to this problem you need to choose another unique attribute 'Dnum' in both the tables as PK and FK correspondingly such that a change of attribute value similar to above will have no effect in the tables which are referring to the department table on DNum attribute.