The WTO Option is an approach to Brexit much favoured by some groupings. It is an approach where the UK leaves the EU without having negotiated any trade agreements with the EU, either within the framework of Article 50 negotiations, or on the margins. Instead, it relies entirely on multilateral WTO agreements covering trade-related matters.
The general thrust of the WTO Option argument is that: "Were the UK to leave the EU, it would continue to have access to the EU's markets, as World Trade Organisation rules prevent the EU from imposing unfair, punitive tariffs on UK exports". In reality, the WTO rules only afford very limited protection against discrimination, and then only in respect of tariffs - which are no longer central to trade matters.
As the WTO site itself says, "by their very nature RTAs (Regional Trade Agreements — as is the EU) are discriminatory", and, under WTO rules, an amount of discrimination against third countries (and that would include the UK) is permitted.
