In blockchain technology, what does the term $"51\%$ attack" refer to$?$

Select one:

a$.$ When $51\%$ of the blockchain's transactions are invalid

b$.$ When $51\%$ of the network$$s data is corrupted

c$.$ When an attacker controls more than $51\%$ of the network$$s computational power

d$.$ When more than $51\%$ of nodes are compromised