The candidates went through an intensive instruction process before their absorption into the service.

'Instruction' refers to teaching or directions, but the context of candidates being formally absorbed into a service requires a word denoting formal initiation. 'Induction' is the process of formally introducing or initiating someone into a new job, position, or organisation, which is exactly what occurs before absorption into the service. 'Immersion' means deep involvement. 'Intimation' means an announcement or hint. 'Unction' refers to anointing with oil. Therefore, 'induction' is the appropriate improvement.