Which of the following expressions is written in scientific notation?

a) 73.4 x 105

b) 0.09 * 107

c) 80 x 103

d) 4.22 x 10-3

Scientific notation is a way to shorten numbers that are way too large or way too small on the other hand. In this case, the number on the left should only be of units digits not containing hundreds, thousands, and so on. Hence the answer to this problem should be D.