Q.The standard deviation of a sample of size 50 is 6.3. Determine the standard error whose population standard deviation is 6 ?

Standard error of the mean $=\dfrac{\sigma}{\sqrt{n}}=\dfrac{6}{\sqrt{50}}\approx 0.85$.

In the TN HSC Class-12 statistical-inference topic, the standard error (S.E.) of the sample mean is computed from the population standard deviation $\sigma$, not the sample value.

Given: $n=50$, population $\sigma=6$ (the sample SD $6.3$ is extra information not needed for the S.E. of the mean based on the known population $\sigma$).

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