Which of the following numbers is written in the standard form ?

The correct answer is 8.5×10^-12.

Understanding Standard Form (Scientific Notation):
A number is written in standard form (also known as scientific notation) when it is expressed in the format:

$m\×{10}^n$

where:
1. $m$ is a number greater than or equal to 1 and strictly less than 10 ($1\≤m\<10$). This means there must be exactly one non-zero digit to the left of the decimal point.
2. $n$ is an integer (positive, negative, or zero).

Evaluating the Given Options:

1. 83.7 × 10^-9: Here, $m=83.7$. Since 83.7 is greater than 10, it is not in standard form.
2. 45 × 10³: Here, $m=45$. Since 45 is greater than 10, it is not in standard form.
3. 8.5 × 10^-12: Here, $m=8.5$. Since $1\≤8.5\<10$, this number satisfies all conditions for standard form.
4. 0.6 × 10¹⁵: Here, $m=0.6$. Since 0.6 is less than 1, it is not in standard form.

Therefore, 8.5×10^-12 is the only number written correctly in standard form.