Is the last name a primary key?

A primary key is always unique and not null. It is prefer to choose a field that is unique like roll number . But to choose Last name as a primary key is not a good option as two people can have same last name.

A Last name field is not a good one for a primary key. A primary key cannot have duplicate values. A last name field is highly likely to have duplicate values, so it should not be used as a primary key.