What is Transient execution attack?

Answer: C. Data of other processing held temporarily in buffers inside the CPU, such as line fill buffers, are passed to instructions that execute only transiently

MDS is a family of transient execution attacks published under names such as RIDL, Fallout and ZombieLoad. On affected CPUs, when a load was cancelled by an exception or similar, data of another process or the kernel left not in the cache but in internal buffers could be passed to subsequent instructions executed transiently, and could be extracted through a cache side channel. The attacker cannot choose the address read but can collect secrets by observing repeatedly. As a countermeasure, microcode updates added a function that clears the internal buffers when switching privilege levels, but another thread using the same core simultaneously through SMT can observe without any switch, so not sharing cores with untrusted parties also became necessary.