Physical Properties of Alkenes

$\left(\mathrm{a}\right)$ $\mathrm{BrHC} -\mathrm{CHBr}$ exists as two diastereomers draw them and compare their dipole moment.
$\left(\mathrm{b}\right)$ trans-Butenedioic acid has a higher melting point than cis-butenedioic acid. Why$?$
$\left(\mathrm{c}\right)$ Draw the cis and trans structures of hex-$2$-ene. Which isomer will have higher b.p. and why$?$

Correct order of stability of following alkenes is-

The most polar compound among the following is:

The correct order of dipole moment is:

P 

Q 

R 

S 

Arrange the following in decreasing order of dipole moments.

I 

II 

III