What Type of Attack Would Be Considered a Very Large Flaw

Multiple Choice

What type of attack would be considered a very large flaw in public blockchains such as Bitcoin's Blockchain where the majority of hashpower could possibly be controlled thru an attack? What is the specific attack Bitcoin could be exposed to?

A) 51% Attacks
B) Tether Token Hack
C) DDoS Attack
D) BIP attack
E) Parity Wallet Attack