How can lack of self-management have a hidden effect on a long-term goal?

Correct answer: Small irregularities can accumulate into a major loss of progress

Long-term goals are usually built through repeated daily actions. Missing one small task may have little effect, but frequent postponement, irregular practice and unmanaged distractions can accumulate into a large gap in learning or preparation. The harm may be noticed only later, which makes it hidden at first. Therefore D correctly describes the cumulative effect of weak self-management.