Both ‘which’ and ‘that’ are relative pronoun, but the ‘which’ is non-restrictive modifier while ‘that’ is a restrictive modifier.
To understand the difference between restrictive and non-restrictive modifier, let’s take an example,
John likes bungalows that have big space in front of them.
In above sentence, the bungalow that John like is limited to only those that have big space in front of them. The ‘that’ clause restrict the noun, hence it is a restrictive modifier. Let’s see the same sentence with which clause,
John likes bungalows, which have big space in front of them.
The above sentence states that John likes bungalows and bungalows have big space in front of them. The bungalows are not limited by the ‘which’ clause. Hence which is a non-restrictive modifier.
Note – In GMAT SC question, using the context of the sentence, you should first try to understand whether restrictive or non-restrictive modifier should be used and accordingly use ‘which’, if a non-restrictive modifier is required or use ‘that’, if a restrictive modifier is required.
Now, look at the below sentence and try to determine whether which should be used or that should be used –
The government has made redevelopment mandatory for buildings, which were built 15 years ago.
If you think the use of which is not right in above sentence, then you are absolutely correct. The above sentence states that government has made redevelopment for all the building mandatory. However, from the context, it is clear that government has made redevelopment for only those buildings that were built 15 years ago. Hence the use of which is wrong. The correct sentence will be as follow –
The government has made redevelopment mandatory for buildings that were built 15 years ago.