The correct option is C Though
A subordinate clause "They built a high wall" is a clause that is dependent on the main clause "the field was flooded when the rains came" for its full meaning. To combine these two as a sentence, a subordinating conjunction is required. When a subordinate clause is the cause for which the main clause is its effect, 'because' is used. Since the given subordinate clause is not the cause, but contrast in meaning to the main clause, 'because' needs to be replaced, hence option A is incorrect. 'Though' is a conjunction used for contrast-related sentences, like the given one. In spite of building a high wall, the result was the opposite, the field still got flooded, hence these are opposite in meaning, and 'though' is appropriate here. Option C is thus, correct. 'As' is similar to 'because', used in cause-related sentences, and the given subordinate clause is not a cause, hence option B is also incorrect. 'If' is used for condition-related related sentences, in which the subordinate clause is the condition based on which the main clause takes place, example, "If they had built a high wall, the field would not have been flooded". Since this isn't the case here, option D is incorrect.