In the given sentence, the correct preposition to use is "in" because it correctly conveys the idea that people who are good at making friends typically work in the field of trade. Let's break down the sentence and analyze it in detail:

1. People who are good at making friends:
- This refers to individuals who possess the ability to establish and maintain positive relationships with others. They are adept at socializing, connecting with people, and building friendships.

2. Usually work in the field of trade:
- This indicates that the preferred profession or occupation for such individuals is in the field of trade. The field of trade encompasses various industries and sectors involved in buying, selling, and exchanging goods and services.

The sentence suggests a correlation between the skills required to make friends and the field of trade. It implies that individuals who excel in social interactions and possess good communication skills are more likely to thrive in trade-related professions. This assertion can be justified by considering the following reasons:

- Building relationships: Trade often involves establishing and nurturing connections with clients, customers, suppliers, and other stakeholders. Good communication skills and the ability to make friends can facilitate the development of strong relationships, which can be beneficial in the trade industry.

- Negotiation and persuasion: Trade frequently entails negotiations, persuading others, and reaching mutually beneficial agreements. People who are good at making friends are often skilled in the art of persuasion, which can be advantageous in trade-related negotiations.

- Networking: Successful trade professionals often engage in networking activities to expand their professional contacts and create business opportunities. Those who are skilled at making friends are more likely to excel in networking, as they can easily establish rapport and build connections with new people.

- Customer relations: In trade, maintaining positive relationships with customers is crucial for repeat business and long-term success. Individuals who are good at making friends are more likely to excel in customer relations, as they can provide excellent customer service and establish loyal clientele.

- Collaboration and teamwork: Trade-related projects often involve collaboration and teamwork. People who are good at making friends are generally adept at working well with others, fostering a cooperative and harmonious working environment.

In conclusion, the sentence suggests that individuals who are good at making friends usually work in the field of trade due to the alignment between their communication skills, networking abilities, and the demands of trade-related professions.