To answer this question, we’ll start by defining what a 51% attack is. It refers to an attack on a blockchain by a group of users that control more than 50% of the network’s hashing/computing power for a PoW system or stake for PoS system. The main concern of this attack is that by controlling the majority of the resources on the network, an adversary can interfere with the process of recording new blocks.
No PoW or PoS system is resistant to an adversary holding 51% of the resource (hashing power or stake respectively), and Cardano as a PoS based system is no different. It is, however, resistant against 34% attacks.
Cardano uses the Ouroboros protocol, which is provably secure under the assumption that there is an honest majority of stake. This is in line with all other cryptocurrencies, whether proof of stake or proof of work - if an adversary were to acquire more than 51% of the resource that determines block creation (hashing power for PoW, stake for PoS) they would be able to take control of the system.
You can find more information on Ouroboros (papers and videos) here: