Answer
Spring A is stronger.
Work Step by Step
Calculate extension for each spring:
Spring A:
Natural length: $4.0\text{ cm}$
Stretched length: $5.4\text{ cm}$
Extension: $5.4-4.0=1.4\text{ cm}$
Spring B:
Increases by $15\%$
Extension$=0.15\times 4.0=0.6\text{ cm}$
Spring B is stronger because it stretches less under the same force.
