Two samples of steel are tested: Sample X contains 0.10 mass % C and has a tensile strength of 400 MPa, whereas Sample Y contains 0.60 mass % C and has a tensile strength of 900 MPa. Which statement provides the best structural justification for the observed difference?
a) Sample Y has a larger grain size, giving the material additional interior volume to withstand stress without breaking.
b) Sample Y contains more interstitial carbon, introducing greater lattice strain that resists dislocation movement and therefore supports a higher load before failure.
c) Sample X has fewer substitutional defects, so its crystal structure is more ordered and consequently stronger.
d) The additional carbon in Sample Y contributes extra delocalized electrons to metallic bonding, resulting in stronger bonds and higher tensile strength.