Complex sentence: The Young's modulus represents the factor of proportionality in Hooke's law, relating the stress and the strain; but this law is only valid under the assumption of an "elastic" and "linear" response.
Split into simpler sentences: The Young's modulus represents the factor of proportionality in Hooke's law, which relates the stress and the strain. However, Hook's law is only valid under the assumption of an "elastic" and "linear" response.