The correct answer is D. established on insecure foundations.
The phrase “built upon sand” is a metaphor for something that is not secure or reliable. In this case, the speaker is saying that the man’s plans are not based on solid ground, and are therefore likely to fail.
Option A, based on inexperience, is incorrect because it does not address the issue of the man’s plans being insecure. The man may be inexperienced, but that does not necessarily mean that his plans are not sound.
Option B, resting on immature ideas, is incorrect for the same reason. The man’s ideas may be immature, but that does not necessarily mean that his plans are not feasible.
Option C, resting on cheap material, is incorrect because it does not address the issue of the man’s plans being insecure. The man may be using cheap materials, but that does not necessarily mean that his plans are not going to work.
Therefore, the correct answer is D. established on insecure foundations.