I need some help to understand how this text with the relative pronoun which should be correctly punctuated. Is option A, B or C correct?
Option A: People usually grate the flesh and add water to it to get coconut milk which is used for cooking curries or soups and making desserts. [No comma used at all.]
Option B: People usually grate the flesh and add water to it to get coconut milk, which is used for cooking curries or soups and making desserts. [A comma is used before 'which'.]
Option C: People usually grate the flesh and add water to it to get coconut milk, which is used for cooking curries or soups, and making desserts. [Commas used before 'which' and before the final 'and'.]
It's sort of difficult sometimes (speaking for myself) to try and determine whether the relative pronoun which is introducing a restrictive or nonrestrictive clause. In the sentence above, would the meaning of the sentence change if I snipped off the 'which' clause? Doesn't People usually grate the flesh and add water to it to get coconut milk still makes sense/carry meaning without the 'which' clause?
Hello, Gilbert!
Whether you should use a comma before "which" depends on whether the meaning of the relative clause is restrictive (picking out a certain type of coconut milk, namely, that used in cooking curries, etc.) or nonrestrictive (indicating simply that coconut milk is used in cooking curries, etc.).
If the relative clause is to have restrictive meaning, it should not be set off with a comma. If the relative clause is to have nonrestrictive meaning, it should be set off with a comma. Since I believe the intended meaning is nonrestrictive, my recommendation is to use a comma after "coconut milk,"
You are right that the sentence without the relative clause ("People usually grate the flesh and add water to it to get coconut milk") makes sense, too, and that is further confirmation that the meaning you intend is restrictive. You are not referring to a specific type of coconut milk, but to coconut milk in general.
As to whether you should use a comma before "and making desserts," that is optional. It is correct to use a comma there, and it is correct not to use a comma there. The comma simply assists with readability. I personally would not use a comma there, but would add "for" before "making":
- People usually grate the flesh and add water to it to get coconut milk, which is used for cooking curries or soups and for making desserts.
Sometimes, a simple sentence like this can also be problematic (for me):
The house, which was blown away by the storm, belongs to my grandfather.
It feels like the relative clause above is merely extra information that can be removed. But wouldn't the sentence lose its intended meaning if the 'which' clause is dropped?
I think it would make sense to use the past tense there ("belonged"), since a house blown away by a storm presumably no longer exists. 
As to whether the relative clause should be set off by a comma, this depends on whether the relative clause is identifying a house for the reader ("Which house belonged to your grandfather?" --"The one blown away by the storm") or simply adding a comment about a house that the reader already knows is being referred to with "the house" ("The house belonged to my grandfather. It was blown away by the storm.")
