A wire stretches by 1.5 mm under a load.

Step 1. (a) $\Delta L \propto L$: $3.0\ \text{mm}$.

Step 2. (b) $\Delta L \propto \dfrac{1}{r^2}$: $\dfrac{1.5\ \text{mm}}{4} = 0.375\ \text{mm}$.

Step 3. (c) $\Delta L \propto F$: $4.5\ \text{mm}$.

Step 4. (d) $\Delta L \propto \dfrac{1}{Y}$: $0.75\ \text{mm}$.