Hi Bilal, good question.
In IELTS you must use good logic and inference. A year stretches from winter to summer, winter being the coldest and summer the warmest, so it is logical that the glacier moves 5 (slowest – winter) to 10 (fastest – summer) inches in the year, therefore the answer must be ‘true’.
Again, remember that IELTS is more than just a test of English, it is also a test of thinking.