The correct answer is A. ±5% of average.
The individual variation between test strength of sample should not be more than ±5% of average. This means that the difference between the highest and lowest test strengths in a sample should not be more than 5% of the average test strength. This is to ensure that the test results are accurate and reliable.
Option B, ±10% of average, is too high. This would allow for too much variation in the test results, which could make them inaccurate. Option C, ±15% of average, is also too high. Option D, ±20% of average, is the highest possible variation that would still be considered acceptable. However, it is still preferable to keep the variation as low as possible to ensure accurate test results.
In conclusion, the individual variation between test strength of sample should not be more than ±5% of average. This is to ensure that the test results are accurate and reliable.