-> to show the pride the workers had in their union
The workers are penniless, yet they still wear their best clothes. It says, "They thought of themselves as representing their union to the rest of the world, therefore a good appearance was important", showing the workers are dressed up because they are representing other people and showing a bad appearance would make them look bad.
The answer is not: "To show the irresponsibility of the workers in the union" because putting on good clothes does not mean irresponsibility.
"To show the workers' interest in fashion trends" does not make sense because that is not why the workers dressed up.
"To show the workers' inability to get weather information" Is also incorrect - the workers are cold because they do not have enough money, not because they don't know about the weather.