प्रकाशिक सक्रिय समावयवी (isomers) जो एक-दूसरे के दर्पण प्रतिबिंब नहीं होते हैं, क्या कहलाते हैं?

Which of the following pairs can be separated by ordinary physical methods such as fractional crystallization?

The number of optically active stereoisomers for 2,3-dichlorobutane is:

Which among the following statements correctly describes a 'meso' stereoisomer?