Which of the following contains an asymmetric centre?

The specific rotation of $(+)$-Mandelic acid is $+158^o$. What will be the specific rotation of a mixture containing $25\%$ $(-)$-Mandelic acid and $75\%$ $(+)$-Mandelic acid?

Which of the following compounds exhibits optical isomerism?

Which of the following molecules contains an asymmetric carbon atom?

Among the following,which compounds can exhibit optical activity?

Provide detailed information about the relationship between optical activity and molecular structure.