Which number contains four significant figures?
3.978
0.780
0.0085
1700
Two or more of the numbers contain four significant figures.

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 \(\boxed{3.978}\).