Right option is (a) Blood vessels become fragile
Best explanation: Fragile blood vessels are not the symptom of Classic Dengue Fever, instead it is a symptom of Dengue Haemorrhagic Fever(DHF). It is a fatal form of dengue fever which starts with fever, nausea, headache, abdominal pain, cold extremities with haemorrhagic spots, internal bleeding leading to blood in stools, urine and vomiting.