What is the primary characteristic of a put option?

The primary characteristic of a put option is that it provides the holder with the right to sell an underlying asset at a predetermined price, known as the strike price, before or at the expiration date of the option. This characteristic allows the investor to benefit from a decline in the asset's price, as they can sell it at the higher strike price while the market price is lower.

This feature is particularly valuable in bearish market conditions or when an investor wants to hedge against potential losses in a portfolio. By exercising a put option, investors can limit their risk and achieve profit in declining markets.

The other options do not correctly represent the essence of a put option. For instance, a call option is what grants the right to buy, while trading without obligations pertains more to the features of options in general rather than specifically to put options. Lastly, regulation of market transactions involves broader market oversight rather than the specific rights conferred by options like puts.