The type of channel that speaker one and speaker two use in their communication would be auditory - b.
The fact that we're talking about "speaker" 1 and "speaker" 2 implies already the fact that both of them are talking and this then implies that they are using the auditory channel of communication.
Tariff makes the most sense. Embargo is a ban on goods or a country, subsidy is money granted from the government to a business and has nothing to do with trade, quota is a limited amount of a product and again doesn't really have anything to do with trade.