which block should be replaced in a cache miss?

block replacement strategies

on cache miss, hardware must select a block to be replaced.

direct mapping; only one block checked for hit, and only one block can be replaced.

with set associative and fully associative, there may be many blocks to choose from.

2 strategies for selecting block to replace:

random (evict at random), or use pseudorandom block selector for easier debugging
LRU (least recently used).

LFU (least frequently used) is not generally one of the methods of block selection.