Enantiomers cannot be distinguished by which of the following?

How many stereoisomeric tetrabromides will be formed in the following reaction?
$1,4-\text{pentadiene} + 2Br_2 \xrightarrow{CCl_4} \text{Product}$

Optically active isomers that are not mirror images of each other are called:

Which one of the following compounds is optically inactive?

Which of the following compounds is an optically active compound?

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?