the correct answer here is still pretty ugly, so let's criticize the four incorrect choices.

choice a:
faulty comparison:
'unlike many u.s. cities, ... the river in san antonio...' --> can't compare cities to a river

choice b:
comparison isn't so good:
the second part should start directly with 'the river' (instead of 'in san antonio the river...'), so that the comparison is more clear.
much more importantly, poor use of the relative pronoun 'which':
'...cities in the united states, which is no longer the focal point...'
relative pronouns like which, by default, refer to the noun that's closest to the left, which in this case is the united states. therefore, this relative pronoun is used in a way that's either ambiguous (if you allow it to potentially refer to other nouns) or just plain wrong (if you follow the rule strictly).

choice c:
faulty comparison again
'the river...' is being compared to san antonio.

choice d:
drastic change in meaning
this sentence has completely gotten rid of any reference to the concept of 'no longer'. while all the other sentences say that things have changed, this sentence presents the situation as if it's always been that way.