The correct answer is C. Until civil engineers consider widening the bridges, these traffic problems will continue.
Explanation:
A closing sentence is usually the last sentence in a paragraph or text that aims at restating or putting, in other words, the main idea of a paragraph or text and/ or providing some conclusion or final thought related to the ideas that were developed during the text. Additionally, closing sentences show the development or explanation or ideas has finished and instead the author is taking the previous ideas to summarize them or provide a conclusion.
In the case of the paragraph presented the author writes about the traffic problems caused by a river as the sections of the roads that are over the river are too narrow and become bottlenecks that cause important traffic issues, also the author mentions the roads can be expanded but if the bridges are not the problem continues. Considering the information provided in the paragraph, a closing sentence for this paragraph will need to summarize the ideas of it, provide some possible solution or some conclusion related to this situation as this will close the ideas of the sentences. Because of this, the best closing sentence would be "Until civil engineers consider widening the bridges, these traffic problems will continue" as this sentence provides a conclusion of the situation explained in the paragraph by using the information developed on it as during the paragraph the author mentions the problem is not the roads but the bridges and thus, if the bridges are not modified the traffic caused by the river will continue.