Validation of drop down by index field

Dear friends

want to validate a field ( drop down by index ) which has 5  values, suppose user enters wrong value by mistake it should throw an error

for eg

the valid values are

'AAA'

'BBB'

'CCC'

'DDD'

'EEE'           

suppose user enters 'XYZ' it should throw an error

how to do validation in this scenario

Thanks

Vijaya