Answer: Humdrum doesn't belong here because it is a word that is used to show lack of motivation or variety.
Sonorous: Word that is used to describe something loud or thundering(?) you obviously can notice that the other words are used to describe something that is physical/ material as the body of a person.
Posh: A fancy word for elegant, elegant doesn't fit when you are trying to salute someone, you can't said "Posh! the day is beautiful".
Careen: Word used to tell that a car is out of the street, the others words are used to describe an object, like cups or boats, big containers.
Forage: Is something related to animals/ people, search widely for food or provisions, the other words are synonyms of persuade( convince others to do something for you).
This one is hard ! I will said that is "Shamble" because this one need to be next to a pronoun to point that this character is moving in a slow, heavy way when the others doesn't need it.
Serif: Word use it to describe the slight projection finishing off a stroke of a letter in certain typefaces.
Explanation: Good Luck!