This question tests vocabulary, specifically the ability to identify synonyms. 'Essential' means absolutely necessary or very important. Among the options, 'important' is the closest in meaning. 'Optional' means not required, 'casual' means informal and not serious, and 'trivial' means unimportant or insignificant. Therefore, the correct answer is B. * **Core Concept:** Vocabulary, synonym recognition * **Solving Strategy:** Identify the word with the closest meaning to 'essential'. * **Option Analysis:** * **A:** Incorrect. 'Optional' means not required, the opposite of 'essential'. * **B:** Correct. 'Important' is a synonym for 'essential'. * **C:** Incorrect. 'Casual' means informal or not serious, unrelated to 'essential'. * **D:** Incorrect. 'Trivial' means unimportant, the opposite of 'essential'. * **Easy Mistakes:** Selecting a word with an opposing meaning or a word that doesn't match the intensity of 'essential'.