Which Google Cloud features could help prevent DDoS attacks?

This answer is correct, because HTTP Load Balancing mitigates and absorbs many layer 4 below attacks such as SYN flood, IP fragment floods, and port exhaustion. CDN caches cacheable content at points of presence close to users. In the event of a DDoS attack for cacheable content, the requests are sent to points of presence, not to your servers/infrastructures, thus increasing the likelihood of the attack being absorbed. Google Cloud Armor is built for DDos mitigation, working with Cloud Load Balancing to detect DDoS attacks.