Which as a relative clause.

Yep. No problem with that one.

Bear in mind that "which" itself isn't the relative clause. It's just the relative pronoun.

The "which" is used correctly. However, there are other mistakes in the sentence.

John got a mark (or a grade) of 75 on the text. which his mum didn't think was good enough.

Okay, thanks guys

But what are the differences between ''John got a mark of 75 in a test, which his mum didn't think was good enough.'' and ''John got a mark of 75 in a test, but his mum didn't think it was good enough.'' and ''John got a mark of 75 in a test, and his mum didn't think it was good enough.'' ?

''John got a mark of 75 in a test, which his mum didn't think was good enough.''

This is the objective voice. It's just relating a fact. It doesn't suggest how anybody felt about it. The use of the relative clause suggests that the content in the clause is non-essential to the sentence.

''John got a mark of 75 in a test, but his mum didn't think it was good enough.''

The "but" suggests that John (or somebody else) thought it was OK, but his mom disagreed.

''John got a mark of 75 in a test, and his mum didn't think it was good enough.''

This is also pretty objective. It just relates the basic information without connection to how anybody felt about it. Using "and" includes it as essential information in the sentence.

Thanks for the explanation.
really appreciate it

No sweat. Keep on learning, eh?

Another question.
''Nowadays things are stored as bytes in computers, which also included our biography.''
Any error in this sentence?

Oh yes.

Yes. First of all, you have a verb tense shift. In the first clause you use present tense, and in the second you jump to past tense for no apparent reason.

Corrected: are stored...include...

Also, it looks like you're using "nowadays" as a transition/conjunctive adverb, so convention says you should use a comma after it to separate it grammatically from the rest of the sentence: Nowadays, things are..."

The other things, I think, can me marked up to awkwardness of expression. Consider these alternatives:

Nowadays, things are stored as bytes in computers, which includes our biography.

Nowadays, things are stored as bytes in computers, including our biography.

my uncle played Santa, he was a relative Claus

Okay, thank you