Final answer:
The number that shows exactly four significant digits among the given options is C (3091). Other options either have a different number of significant figures or involve trailing zeros that might be considered as placeholders rather than significant figures.
Step-by-step explanation:
The student asked to choose the number that shows exactly four significant digits from the given options. Option C (3091) is the correct answer as it contains four digits, and all of them are significant. In comparison, Option A (4,509,300) has five significant figures, Option B (0.001) has only one, and Option D (1.000) has four, but the trailing zeros may be considered as placeholders depending on context, which could cause confusion. Since the question specifies 'exactly' four significant figures, 3091 is the unequivocal choice.
In determining significant figures, zeros are significant except when they serve only as placeholders. For example, in a number like 15,450.0, all the non-zero digits and the zero after the decimal point are significant, giving us six significant figures, whereas in 0.053, the zeros are not counted since they are merely locating the decimal point, thus having only two significant figures.