(6.022 × 1023) × 2.58 =

Multiply the numbers: 6.022 × 2.58 = 15.53676, so the product is 15.53676 × 10²³ = 1.553676 × 10²⁴. The factor 2.58 has three significant figures, so round the result to three significant figures: 1.55 × 10²⁴, choice A.

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