Which of the following elements should be added to improve the yield strength and tensile...

Manganese is the primary alloying element used in low-carbon steels to increase yield and tensile strength through solid solution strengthening and by promoting grain refinement. It also improves hardenability and removes sulfur by forming manganese sulfides.

Step-by-step Derivation:
Step 1: Analyze the requirements for low-carbon steel. Low-carbon steels (typically < 0.25% C) have low strength and hardness. To improve these properties without significantly increasing carbon (which would reduce weldability and ductility), alloying elements are added.
Step 2: Evaluate Option A (Manganese). Mn is added to almost all low-carbon steels. It increases strength via solid solution strengthening and prevents the formation of hot-shortness by reacting with sulfur. It significantly raises the yield point.
Step 3: Evaluate Option B (Chromium). While Cr increases strength and hardness, it is primarily used for corrosion resistance (stainless steel) or high-temperature strength (alloy steels), and is not the standard primary additive for basic low-carbon steel strength improvement.
Step 4: Evaluate Option C (Tungsten). W is used primarily in tool steels for high-temperature hardness (red hardness) and is too expensive and specialized for general low-carbon steel strength improvement.
Step 5: Evaluate Option D (Nickel). Ni improves toughness and impact strength, especially at low temperatures, but is not as effective or economical as Mn for increasing basic yield and tensile strength in standard low-carbon grades.
Step 6: Conclusion. Manganese is the most common and effective element for the specific goal of improving yield and tensile strength in low-carbon steel.