Well-planned passenger-car parking can increase the use of mass

Premise :Well-planned passenger-car parking can increase the use of mass transportation, reduce the number of commuters, and encourage alternative travel methods.

Conclusion : It's important that communities invest in improvements in passenger-car parking

The main assumption is that passenger car parking can improve all the above if communities invest in it.

A. Mass transportation is becoming more popular as gasoline prices continue to increase => There is comparision or relation stated on these two..Transport vs gasoline prices.
B. Many cities that invested in new parking facilities have witnessed increased usage of mass transportation =>This is like to result of some implementation but not an assumption.
C. There are other ways to increase the use of mass transportation , reduce the number of commuters , and encourage alternative travel methods. => If there are other ways to increase then good parking option is not considered.
D. Some fraction of the prospective increase in the use of mass transportation , and reduction of the number of commuters can be realized => As per our prethinking..yes there can be some improvements if they invest....
E. Parking is the most critical issue for most local communities => This is irrelavant and this is not at all discussed..

IMO option D.