Relative expansion dL/L isequal to alpha*dT, where alpha is the coefficient of linear expansion and dT is the temperature increment.
L = 100 m
dl = 4 cm,
dT = (43.5 - 20) °C = 23.5 °C
Thus, we can find
alpha = dL/(L*dT) = 4*10^(-2) m / (100 m * 23.5 °C) = 1.702 * 10^(-5) 1/°C.
By referring to a table of coefficients for different metals (e.g. http://en.wikipedia.org/wiki/Thermal_expansion
) we find that it's closest to that of copper. Answer: copper