I will join your classes ___________ Saturday.

For

Since

From

None of the above

All of the above

Preposition means before a position.

A word placed before noun or a pronoun to show its relation to some other word in the sentence.

From = point of time (present, past & future)

I will join your classes from Saturday means میں ہفتے سے آپ کے کلاسز جوائن کروںگا۔