Final answer:
A repeating decimal is a decimal number in which a digit or a group of digits repeats infinitely. Option A, 0.6666666, is an example of a repeating decimal.
Step-by-step explanation:
A repeating decimal is a decimal number in which a digit or a group of digits repeats infinitely.
Option A, 0.6666666, is an example of a repeating decimal because the digit 6 repeats infinitely.
Option B, 0.123321, is not a repeating decimal because the digits do not form a repeating pattern.
Option C, 1.234545, is not a repeating decimal because the digits do not repeat.
Option D, 6.78, is not a repeating decimal because it does not have any repeating digits.
Learn more about Repeating decimals