The correct option is C Trade
In the sentence, 'pick up' means 'to improve or increase' something. The noun we choose from these options is dependent on this phrase 'pick up', 'trade will pick up' means that when tourists arrive, trade will increase. It's a fact that tourists increase the economy of the place they're in, as they spend on food, travel and shopping. Hence option C is correct. 'Suitcases' can't 'pick up', as it means 'suitcases will increase', which is irrelevant. It's confused with 'pick up the suitcases', which means 'lift the suitcase, carry it'. The phrase 'pick up the suitcase' and 'the suitcase will pick up' mean completely different things. A suitcase cannot 'improve', hence option A is incorrect. 'Foreign accents' also cannot increase, or improve, when tourists arrive here, there's no connection between the two. The tourists are foreign, they're the people with a foreign accent, and they cannot improve their foreign accents when they arrive, hence option B is incorrect. 'Weather' is a natural phenomenon, it does not depend on the people, whether they're tourists or locals, hence option D is incorrect.