The computer that I have at work is very slow [ a) defining clause b) non-defining clause 	a)<br />✓ It's the computer which|that I have at work which|that is very slow (but the one which|that I have at home is fast<br />no commas + that → defining cause<br />= no mistakes
Anti-viruses, which have been available for two decades, are efficient [ a) defining clause b) non-defining clause 	b)<br />✓ Anti-viruses (in general) are efficient<br />commas + which → non-defining cause<br />= no mistakes
Only anti-viruses which have been around for two decades are efficient [ a) defining clause b) non-defining clause 	a)<br />✓ It is only anti-viruses which|that have been around for two decades which|that are efficient (and not others)<br />no commas + which → defining cause<br />= no mistakes
The article, that was discussed last week, was more controversial than this one [ a) defining clause b) non-defining clause 	a)<br />〆 obvious mistake (that cannot be preceded by a comma!)<br />'that was discussed last week' = important<br />→ defining clause<br />→ use of commas before 'that' and after 'week' is a mistake<br />correct answer = The article that was discussed last week was more controversial than this one
The company which was founded in 1991 has remained a family business [ a) defining clause b) non-defining clause 	b)<br />is probably lexically correct and the desired answer<br />but a) is grammatically correct...<br />〆b = confusing meaning<br />'which was founded in 1991' = not important (unless other companies have been mentioned before)<br />→ non-defining clause<br />→ absence of commas before 'which' and after '1991' is (most probably) a mistake (without more context).<br />→ for clarity, you could use 'that' to show it is a defining clause and not a non-defining clause.<br />correct (or recommended) answer = The company, which was founded in 1991, has remained a family business