Explanation

Creating a filter: We read the sentence and we're not sure whether there is an error. So, we'll start with answer choice (B) and look for objective defects.

Finding objective defects: the grammar of choice (B) indicates that the festival is doing the encouraging, contrary to the intended meaning, so B is out. In (C), the word "when" is off, because Songkran properly is a festival, not a time. (D) is similar to (A). (E) is out because "by encouragement" is not a proper English expression.

So we're down to (A) and (D). Choice (D) has simple logic with cause and effect established by "since." It uses passive voice, but so does (A), and this is a case where passive voice makes some sense, because we don't really know who is doing the encouraging; it's a general phenomenon. Choice (A) uses the words "in that," which, on closer look, doesn't describe precisely the relationship between the levity and the water-throwing. The phrase "in that" is used to provide more detail or rephrasing something, but that's not the meaning here. The intended meaning is to give reasons and causes, so "since" is better. The correct answer is (D).