My banana new leaves are yellow with green veins and bunched at the top, which nutrient?

Yellow young leaves with green veins point to a micronutrient shortage in the new growth, and in banana the usual suspects are iron, manganese and zinc, which can be hard to separate by eye. If the youngest leaves are bunched, small and crowded at the crown with a rosette look, that crowding leans towards zinc deficiency, while a clean green-vein-on-yellow net leans towards iron, and a more mottled grey-flecked interveinal yellowing leans manganese. Because these overlap, a leaf tissue test is the dependable way to know which one, so please confirm before treating, and rule out waterlogging and root nematode damage, which are common in banana and cause similar pale, stunted growth. Correction is by foliar micronutrient sprays of the confirmed nutrient: ferrous sulphate for iron, zinc sulphate for zinc, manganese sulphate for manganese, all at low concentrations like around 0.5 percent, repeated a couple of times on the new leaves. Many growers use a balanced micronutrient mix only after a test shows what is actually low. Adjust to your tissue test and local KVK or horticulture advice and do not over-apply.