The lowest grade that earns college credit varies by institution and sometimes even by department or program within an institution. There's no universal standard. However, a grade of
D is the most common lowest passing grade that will earn college credit at many institutions. Some may use a D-, and a few might even accept a grade of F for credit in certain circumstances (though this is unusual).
It's crucial to check the specific grading policy of the college or university in question. The student handbook or registrar's website will usually outline this information clearly.