○　正解です。

The prefix 'oct' means eight. Some other common numerical (number) prefixes are : uni 1, du/bi 2, tri 3, quad/tetra 4, sept 7, dec 10.

If oct means 8, why is October the tenth month?

In the original Roman calendar there were only 10 months & 304 days. The year began in March, the month of the spring equinox (春分）and finished at the end of December. The 61 days between the end of December and the beginning of March had no month!
So Septemer was the seventh month of the year, October the eight, November the ninth and December the tenth.