The correct option is C Youth
Concrete nouns name things that can be perceived by any of our senses, sight, touch, taste, smell and hearing.
nouns name a state of being, a feeling, or an idea that cannot be seen,
touched, smelled, tasted or heard. Abstract nouns do not have a
physical form like concrete nouns do.
Option A: Youth is a stage of life, a period between childhood and adulthood. It is a state of being, hence it's an abstract noun.
Option B: Oak is a tree, it is a concrete noun.
Options C and D: Similarly, vase and hourglass are also concrete nouns, the former is a decorative bowl and the latter is a device that measures time. Both can be seen and touched.
Hence options B, C and D are incorrect.