The standard form of the number 12345 is

For the standard form notation, only one number is kept before the decimal point and rest all numbers are written after decimal point.

The power of 10 denotes the number of terms shifted after the decimal point.

So for 12345,

Here only one number will be kept before decimal point that is 1.

12345 = 1.2345 × 10⁴

So the correct answer is D.