What is the difference between write back and write through?
a. with write back, main memory is updated immediately, but with write through, the memory is updated only when the block is replaced.
b. with write through, main memory is updated immediately, but with write back, the memory is updated only when the block is replaced.
c. with write through, the block is replaced when it is written, but with write back, it is replaced when it is read.
d. with write through, the block is replaced when it is read, but with write back, it is replaced when it is written.
e. they are two names for the same thing.

The difference between write back and write through is With write through, the block is replaced when it is read, but with write back, it is replaced when it is written.

D. With write through, the block is replaced when it is read, but with write back, it is replaced when it is written.

Explanation:

Write back and write through difference  as follows.

Write back update the memory cache at specific intervals times with specific conditions. Memory cache is update on corresponding memory location, which makes faster to desktop or workstation or tablet.

Written through is copied caching techniques data copied to higher level caches. Basically it copy from memory cache only. In write through mode just refresh main memory.

So write back update memory cache on specific intervals whereas write through copy the cache and make memory refresh with fresh updates.