ANS:C - carbon

The solute atoms which cause the yield point phenomenon in mild steel are carbon. The yield point phenomenon in mild steel is primarily caused by the interaction of carbon atoms with dislocations in the crystal lattice structure of the steel. When mild steel is subjected to stress, dislocations move through the lattice until they encounter carbon atoms. The carbon atoms act as obstacles to dislocation motion, causing the material to exhibit a sudden drop in stress and a corresponding decrease in strain, known as the yield point phenomenon. While other elements like aluminium, boron, and nitrogen may also affect the properties of steel, carbon is the most significant alloying element in mild steel and plays a crucial role in determining its mechanical properties, including yield strength, hardness, and ductility.