Concept:The Ishihara test is a medical screening tool for color vision deficiency, specifically red‑green colour blindness.Explanation:The test uses plates with coloured dots hiding numbers or patterns. Only people with normal colour vision can see them. Those with red‑green deficiency fail to identify the hidden figures. This makes it a standard test for colour blindness. Other options: Bone density is checked by DEXA scan, IQ by intelligence tests, haemoglobin by blood test.Answer:Colour blindness (Option C).