The institutions and persons covered by this Directive shall apply customer due diligence measures in the following cases:
(a) when establishing a business relationship;
(b) when carrying out occasional transactions amounting to EUR 15 000 or more, whether the transaction is carried out in a single operation or in several operations which appear to be linked;
(c) when there is a suspicion of money laundering or terrorist financing, regardless of any derogation, exemption or threshold;
(d) when there are doubts about the veracity or adequacy of previously obtained customer identification data.