Free state French training, prescribed when you sign the integration contract.
Details
Most people receiving a first French residence permit sign the Contrat d'Intégration Républicaine, and OFII then tests your French and prescribes a free course sized to your level — several hundred hours if you are starting from scratch, none at all if you already test above the threshold. It is the single biggest free French offer in the country and you do not apply for it so much as get assigned to it at your OFII appointment. The course targets everyday and administrative French rather than an external diploma, so pair it with a DELF if you need a certificate for naturalisation.