A millisecond is . . . . . . . . of a second.

The correct answer is A. 1/1000.

A millisecond is one thousandth of a second. It is a unit of time used to measure very short periods of time. For example, the blink of an eye lasts about 300 milliseconds.

The other options are incorrect. Option B, 1/100, is a hundredth of a second. Option C, 1/10, is a tenth of a second. Option D, 1/10000, is one ten-thousandth of a second.

I hope this helps! Let me know if you have any other questions.