Doubt 1

Cl₂, it is an addition agent, not a substitution agent.

Answer is (c)

Can't be. PCl₃ works for all alcohols which are not sterically hindered, as in presence of pyridine it gives S_N2 reaction.

okay Pritish, I trust u .
Anyways thankyou [4]

Yes you're right, this is when pyridine is not used. S_Ni occurs like you have said!

see some energy is needd to break cl-cl which is quite strong,so the given rxn with a is energetically not favoured.

then (accordin to u) answer should be none of these.

But halogens can be added to a double bond system.
It involves breaking of the bond between tem.

Cl2 --> chloronium ion.
Again the other ion adds anti (wise) to form the trans vicinal alkane

so it is (a)