Final answer:
The correct scientific notation for a 20-digit number is 'a digit between 1 and 9' followed by a decimal point and the rest of the significant digits, multiplied by 10 raised to the 19th power, thus the correct answer is a) 2.5 × 10¹⁹.
Step-by-step explanation:
The student asked to choose the correct option describing a 20-digit number in scientific notation. Scientific notation is a way to express very large or very small numbers by using powers of ten. Each option represents a number but to determine if it is a 20-digit number, we need to look at the exponent of 10.
The correct scientific notation for a 20-digit number would be 'a digit between 1 and 9' followed by a decimal point and the rest of the significant digits, multiplied by 10 raised to the power of 19 (since we start counting digits from the hundred trillionth place when representing a 20-digit number).
Let's review:
- The option a) 2.5 × 10¹⁹ shows a number that has 20 digits when expressed in standard notation.
- The options b), c), and d) represent numbers that are larger than a 20-digit number because their exponents are higher than 19.
Therefore, the correct option that describes a 20-digit number in scientific notation is a) 2.5 × 10¹⁹.