a fuse wire repeatedly gets burnt when used with a good heater. it is advised to use a fuse wire of

The correct advice is to use a fuse wire of more radius (greater thickness).

Why a thicker fuse wire?

• The resistance RRR of a wire decreases when its radius (thickness) increases, because resistance is inversely proportional to cross‑sectional area.

• Lower resistance means less heating for the same current, so the thicker fuse wire can carry the heater’s current without melting repeatedly.

• A thin (small radius) fuse wire has higher resistance, heats up more, and therefore burns out again and again even when the heater itself is in good condition.

So, among the options “more length, less radius, less length, more radius,” the correct choice is: more radius.

Information gathered from public forums or data available on the internet and portrayed here.