Liverpool has slowly emerged from its post-industrial decline

Interesting !
firstly, "investors interested in large urban renewal projects than would have been thought possible twenty years ago. "
has a comparative 'than'.
so 'investors' should also be preceded by a comparative adjective.
'Greater investors' is incorrect as investors is countable. we need 'more investors'. Leaves us with A and B. 'Of more' in B is incorrect as it will refer to 'population' illogically.
Therefore A it is.
With is common to the two NOUN items of the list as follows-
Liverpool has slowly emerged from its post-industrial decline as a newly thriving city with
1. a population of over five hundred thousand
and
2. more investors interested in large urban renewal projects than would have been thought possible twenty years ago.