Google Cloud Certified Associate Cloud Engineer Practice

The most likely cause of a core project's deletion in a startup company is due to the project not being properly attached to the organization. Option A, being a victim of malware, does not have a strong likelihood as most startup companies have security measures in place to prevent such attacks. Option B, an engineer intentionally deleting the project out of spite, is not a common occurrence and would likely result in consequences for the engineer. Option D, Google deleting the project due to a failed payment, is also unlikely as companies typically have a grace period for missed payments before taking action. Therefore, option C is the most plausible explanation for a deleted core project in a startup company. It is important for startup companies to have proper organization and management of projects to avoid this issue.