Final answer:
The correct answer to fill in the blank is irony.
Irony is a literary device that involves using words or situations to convey a meaning that is the opposite of its literal or expected meaning. In this context, the statement is describing a recurring pattern where each generation is accused of laziness by the previous generation, only to later do the same to the next generation. This ironic cycle is referred to as the irony of life.
Step-by-step explanation:
The phrase 'Every generation is accused of slacking by the preceding ones, before in turn calling its own progeny lackadaisical; such is the irony of life' best fits the blank in the question. This cyclical pattern of criticism across generations touches upon the humorous and perhaps incongruous nature of human behavior, where each generation fails to recognize its similarity to the previous one when it comes to judging the next. The use of 'irony' is appropriate as it refers to the situation that is contrary to what was expected or intended, capturing the essence of intergenerational perspectives.