The best response is:

b. Speed up and go straight into the grocery store that you can see a block ahead of you.

The best thing that you can do in that situation is run for the nearest open business, whether it is a restaurant or store. You can go immediately to the store manager or employee and ask to use the phone to call the police. If you stop to yell at the person or to pull out your phone to call your friend, then you give the other person more time to get to you and possibly harm you. Don't be afraid to yell while you are running so that you can get the attention of other people who may help you.