What causes accents to form

The environment and language spoken where the person is brought up.

For example, if someone in brought up in NY or North Jersey, they would hear people pronounced "water" as "woder" (NY) or "wudder" (NJ). They would adapt this, causing an accent to form.

If someone is trying to transition from a different language, their first attempts at the new language will sound quite funny because of their heavy accent from their first language.

b is the right answer for this question.