The latter one is correct.

land area is comparable to that of Great Britain is correct usage

Compare x to that of y.

Then whom implies people here, while Option A sounds ambiguous!

it's a good question though on parallelism and idiom usage!

Laos has a land area about the same as Great Britain but only four million in population, where many are members of hill tribes ensconced in the virtually inaccessible mountain valleys of the north.

A) comparable to the size of Great Britain, but only four million in population, and many
B) comparable to that of Great Britain but a population of only four million people, many of whom

Option A: Laos has a land area comparable to the size of Great Britain, but only four million in population, and many are members of hill tribes ensconced in the virtually inaccessible mountain valleys of the north.

Option A compares the land area of Laos to the size of Great Britain; since the verb used is 'comparable' that means that you have to compare to similar objects

Option B: Laos has a land area comparable to that of Great Britain but a population of only four million people, many of whom are members of hill tribes ensconced in the virtually inaccessible mountain valleys of the north.

Option B clearly describes that the land area of Laos is comparable to the land area of Great Britain by inserting the pronoun "that" in the answer choice

I'd say the latter one too, because in option A we don't know who does many refers to, but in option B it's referring to the Laos's people.