One C will not keep her from getting into many wonderful schools including Cal Poly SLO especially if she has shown better grades in her subsequent rigorous Math courses. As noted by @eyemgh, there is nothing that can be done at this point and CSU application does NOT allow submission of additional comments to address the 9th grade Covid situation. Just realize that she was not alone and many students had difficulty with on-line/distance learning during Covid.
Cal Poly SLO like all the CSU’s admit by major, are heavily stats based and admission chances are dependent upon an applicant’s intended major.
My advice is that she not focus on any particular school but approach her college list from the bottom up. Find a couple of Very Likely schools that are affordable, offer her major(s), where she will happily attend and are affordable. There are many colleges that would be happy to have her as a student.
If she is unsure of major, please note that SLO does not offer Undeclared but there are many CSU’s that allow a student to apply as Undeclared and where she can explore her interests.
In regards to Undeclared, Competitive/Impacted majors should be selected as a first choice since there is no guarantee that a student will be able to declare these majors later if going in as Undeclared.
She should not try to explain or make excuses for 1 grade (which is passing). She needs to find schools that fit her stats and not the other way around.