Step 1 :Significant figures are the digits in a number that carry meaningful information about its precision. In general, all non-zero digits are considered significant. For zero digits, any zeros between significant digits are significant, zeros to the left of all non-zero digits are not significant, and zeros to the right of all non-zero digits are significant if they are at the end of the number and the number contains a decimal point.
Step 2 :Let's analyze each number:
Step 3 :3.978: This number has four digits and all of them are non-zero, so all of them are significant.
Step 4 :0.780: This number has three digits and all of them are non-zero, so all of them are significant.
Step 5 :0.0085: This number has two non-zero digits which are significant and two zeros to the left of non-zero digits which are not significant.
Step 6 :1700: This number has two non-zero digits which are significant and two zeros to the right of non-zero digits which are not at the end of the number or the number does not contain a decimal point, so they are not significant.
Step 7 :So, the number 3.978 contains four significant figures.
Step 8 :Final Answer: The number that contains four significant figures is \(\boxed{3.978}\).