The diameter of the hair is 0.000072 meters. To convert this number into scientific notation, we move the decimal point to the right until we have a number between 1 and 10.
Moving the decimal 5 places to the right gives us:
\[ 7.2 \]
Since we moved the decimal 5 places to the right, we represent this as:
\[ 7.2 \times 10^{-5} \]
Therefore, the correct answer is:
7.2 x 10^-5.