Contingent Claim

Is it correct to say that the buyer of the option can default?. Not buying the option is defualting

The buyer of an option cannot default. But the writer of the option can default if he does not buy the asset when it is exercised. If options are traded on a regulated exchange, then a put writer would need to post margin but a put buyer would not need to because they are not required to actually exercise the put if it is out of the money.