It appears that the given sentence aims to communicate the idea that under certain conditions (if it is needed or necessary), new words can or should be employed to fully express the intensity or emotion of an exclamation. The key elements here are (i) conditionality (the use of new words is not absolute; it depends on whether it is necessary) and (ii) purpose (the goal is to express the "full force of exclamation"). Let us evaluate the given choices - 

Option A: Here, "could be used" is less definitive and slightly informal - it suggests possibility rather than recommendation, which weakens the intended meaning. Furthermore, the conditional phrase ("if required") is awkwardly placed in the middle of the sentence, disrupting the flow and making it less clear.

Option B: This choice corrects the problem with Option A: the phrase "if necessary" is placed at the beginning, making it immediately clear that the use of new words is conditional; this sets up the context. The use of "should be used" conveys a sense of recommendation or appropriateness, which fits well here. The sentence follows a natural order: condition → action → purpose. This structure makes it easy to follow the reasoning:

Option C: The structure here is awkward and confusing. The purpose ("to express the full force of exclamation") is stated first, followed by the condition ("if necessary"), and then the action ("new words will be used"). This disrupts the logical flow and makes the sentence harder to follow. Moreover, "will be used" is overly definitive and does not convey the conditional aspect as effectively as "should be used."

Option D: There seems to be a tense inconsistency here: "are used" is present tense and suggests a general truth or habitual action rather than a conditional recommendation. This makes the sentence less precise and less compelling.

Hence, Option B is the best choice.