Which measurement would be least likely to be written in scientific notation: the thickness of a dog hair, the radius of a period on this page, the ounces in a cup of milk? explain your reasoning.

None of them NEEDS to be written in scientific notation, IF the best unit is
selected to represent each one ... micrometers for the hair and the period, etc.

For the cup of milk, you specified the unit of measure.  Using that unit,
the number is more than 1 and less than 10 , so there's no need at all
for scientific notation.

(800 x 9) + (10 x 9)

Step-by-step explanation:

Find out what 810 x 9 is:

810 x 9 = 7290

Then see which expression has the same answer. Personally I'd check all of them, but luckily the first expression is the correct one so you don't need to go through the rest (since it only asks for one correct expression):

(800 x 9) = 7200

(10 x 9) = 90

7200 + 90 = 7290