ANS:D - 400 kg/cm2

In favorable circumstances, a 15 cm concrete cube after 28 days typically attains a maximum crushing strength of around 400 kg/cm². Concrete gains strength as it cures over time, and the 28-day compressive strength is commonly used as a benchmark for assessing the quality of concrete. Under optimal conditions such as proper curing, adequate mix proportions, and suitable aggregates, concrete cubes can achieve compressive strengths ranging from 300 kg/cm² to 500 kg/cm² at 28 days. While 400 kg/cm² is a commonly observed value for high-quality concrete, the actual strength achieved can vary based on factors such as the specific mix design, curing procedures, and environmental conditions during the curing period.