The correct option is B well-trained
A compound adjective is made up of two words, linked with a hyphen. 'Well-trained' is a compound adjective made up of two adjectives, 'well' and 'trained' that are joined to modify the same noun 'dog', the hyphen is used to avoid ambiguity in the meaning of the sentence. Hence option B is correct. If the hyphen is removed, the meaning of the sentence changes and if any one adjective is omitted, the sentence sounds incomplete. Option A is incorrect because it's a noun. Option C is incorrect because Option A is not an adjective. Option D is incorrect because option B has a compound adjective.