Final answer:
The diameter of a microbe, 0.00000251 meters, is correctly expressed in scientific notation as 2.51 x 10^-6 meters, which corresponds to option A: 2.51x10-6.
Step-by-step explanation:
To express the diameter of a microbe in scientific notation, you first need to write the number in such a way that there is one nonzero digit to the left of the decimal point. For the number 0.00000251 meters, we write 2.51. Then, count how many decimal places you need to move the decimal point to get from the original number to the new format (2.51). In this case, we move it 6 places to the right, so we use a negative exponent because the original number is less than one. Therefore, the number in scientific notation is 2.51 x 10-6 meters. Given the choices, the correct answer is A: 2.51x10-6.