Ok, I did a quick search on the Hertz.de website and this is what they have displayed. They do not have the model names on the German website, so I cross-checked with the French website doing a dummy booking on Frankfurt airport in November
Group O Ford Mondeo or similar: Citroen C4, Ford Mondeo, Opel insigna, Ford Grand C-max
Group N Peugeot 3008: Peugeot 3008, Nissan Qasqai, Ford Kuga
Hope this helps!
FYI, I've found that the French website is quite useful for "or similar" models, with help of Google Translate you should be able to check on your booking place and time. Since production has stopped, if your booking is next year there's little chance to get a C-max indeed