The correct answer is B. projecting.
To project means to plan or estimate the future course or development of something. In this case, the speaker is saying that the listener will have to face some practical problems when they start planning or estimating the future course or development of this plan.
The other options are incorrect because they do not make sense in the context of the sentence.
A. prosecuting means to take legal action against someone. This does not make sense in the context of the sentence, as the speaker is not talking about taking legal action against someone.
C. prescribing means to give orders or instructions. This does not make sense in the context of the sentence, as the speaker is not giving orders or instructions to the listener.
D. proscribing means to forbid or prohibit something. This does not make sense in the context of the sentence, as the speaker is not forbidding or prohibiting the listener from doing something.