Indian steel designation (IS): According to Indian standards, the plain carbon steel marked as 40C8 indicates a carbon content in the range of:

Introduction / Context:
Several national standards encode steel composition in alphanumeric designations. In the Indian system for plain carbon steels, the digits and letters convey approximate carbon and manganese levels, aiding quick selection on shop floors and in design documents.

Given Data / Assumptions:

• Designation: 40C8 (plain carbon steel).
• Convention: the number before the letter relates to carbon content.
• The trailing number is associated with manganese level (approximate).

Concept / Approach:
For 40C8, the “40” signifies approximately 0.40% carbon (typically a range around 0.35–0.45% to allow tolerance). The “C” denotes carbon steel and the “8” indicates about 0.8% manganese. Thus the correct carbon range to associate with 40C8 is 0.35–0.45%. Other listed ranges or single values either understate or overstate the carbon content relative to the standard meaning of “40”.

Step-by-Step Solution:
Interpret first two digits: 40 → ~0.40% C.Translate to a permissible range: typically 0.35–0.45% C.Ignore confusing alternatives that do not match the code.Choose 0.35 to 0.45% as the best answer.

Verification / Alternative check:
Common handbooks list 40C8 with C ~0.40% and Mn ~0.80%, aligning with medium-carbon structural steels.

Why Other Options Are Wrong:
0.04% is much too low; 0.6–0.8% is tool-steel range; 0.4–0.6% is too wide and not the standard tolerance band; 0.08% misreads the designation.

Common Pitfalls:
Confusing the trailing “8” as carbon content; assuming exact single-value composition rather than a practical range.

Final Answer:
0.35 to 0.45%