The correct answer is B. flower. A concrete noun is something that can be experienced by the 5 senses. All of the other noun choices are concepts; you cannot touch love or taste love, but it is there. You can't smell respect, but it is there. Since a flower is a physical object and the other choices are abstract nouns, flower would be the correct answer.
Hope this helps! Kudos to Miroir for correcting the wrong answers as well :)