We can use this phrase when we can’t think of an appropriate answer to a question. It means that you are thinking about the answer but you don’t have it immediately. It is perfectly acceptable to say this in the IELTS speaking exam but you must come up with an answer after saying it. It is better than “confused” silence. Read this post for more help on giving yourself time to think in the IELTS speaking exam.
Examiner: What are the main factors that have contributed to the economic growth of your country?
Candidate: Hmmmm. Nothing springs to mind immediately. I suppose our production of high quality electronics has contributed …….. (etc.)