The question tests the idea of nutrient mobility inside a plant, and how it decides whether a deficiency shows up in old leaves or young leaves first. Mobile nutrients get shifted out of old leaves toward new growth, so old leaves show the symptom. Immobile nutrients cannot be moved this way, so young leaves suffer instead.
- Sulphur: shows some resemblance to nitrogen deficiency and has limited mobility, but it is not the standard textbook case used for young-leaf symptom questions.
- Iron: fixed once used inside mature leaf cells and cannot be pulled back out and resent to new leaves. Because of this, iron deficiency shows up as interveinal chlorosis in the youngest leaves first, the classic example of an immobile nutrient.
- Nitrogen: highly mobile, withdrawn from older leaves and sent to new growth when scarce, so its deficiency appears in old leaves first.
- Potassium: also highly mobile, moved from older tissue to younger tissue when scarce, so its deficiency again shows in old leaves first.
Since the question describes symptoms appearing first in young leaves due to immobility, the answer has to be the immobile element among the four, which is Iron.
Let's summarize:
- Mobile nutrients, N and K, show deficiency in old leaves first.
- Immobile nutrients, like Fe, show deficiency in young leaves first, since they cannot be recycled from mature tissue.
So the correct answer is Iron.