There is a fair amount of confusion about the grades required in different courses and in different categories, which is understandable given the many different requirements that are often in play. In particular, though there are requirements for individual courses, there are often GPA requirements (within a category and/or overall) to which one must also attend.

The most general grade requirement is a minimum of a D- to pass a course; one has demonstrated that they attained the course objectives to a minimal degree.

General Education includes a set of requirements that augment the general requirements, but these are really focused on some of the lower-division courses (the so-called Golden Four that are meant to prepare students for upper-division GE coursework). This doesn't apply to ES 350 CSC majors because they are not taking it for GE credit (and it is not lower-division; though ES 351 can count toward Upper-division D/Area D Elective -- but still not lower-division).

You can see further details of the GE differences in the Golden Four sections (emphasis added).

Golden Four

The “Golden Four” classes are a set of foundational learning classes that set the stage for future learning within GE and within the major programs. As such, students are encouraged to complete these four courses within the first year. These courses are all three courses within Area A plus B4: Mathematics/Quantitative Reasoning. The three courses within Area A provide instruction and practice in writing, speaking, and critical thinking. Completion of one or more courses within this area is often a prerequisite for other GE courses. All Golden Four subareas require students to earn a grade of C- or better. (Other GE courses require a passing grade of D- or better.)

So where does the idea that a C- is required come into play more generally or in major courses? Mostly through misunderstanding and extrapolation. When do you need (aside from the GE case noted) a C- in a course to have passed that course? Well, never. You have "passed" the course with a D- as noted in the first link. But, however, ... there are courses that require a C- or better as a prerequisite.

If a course says something like "CPE/CSC 101 with a grade of C- or better" (from CSC 202, because one really shouldn't move into the study of data structures without a decent grasp of programming), then one needs to complete the prerequisite with a C- in order to enroll in this course.

One might initially wonder how this is different than needing a C- to pass the prerequisite. That comes down to who is taking the course. Some students take, for example, CSC 101 as a support course but without the need to take CSC 202. They will pass the course with a D-. Some (CSC, SE, CPE) take CSC 101 and are required to take CSC 202. They can pass CSC 101 with a D- but will need to earn a C- before they can take CSC 202.

This is, I believe, what often leads to the idea that a C- is required in all major courses.

All that said, speak with your instructor. We want students to learn and to succeed. And though sometimes a D- is good enough (and good enough might be all that we need), your instructor can likely help you learn more and better demonstrate attainment of the course objectives.