The correct answer is A. since.
The word “since” is used to indicate a point in time in the past from which something has been happening or continuing. In this case, the speaker is saying that they have not ridden a bike since a point in time in the past.
The word “for” is used to indicate a period of time during which something happens or continues. In this case, the speaker is not saying that they have not ridden a bike for a period of time, but rather that they have not ridden a bike since a specific point in time.
The word “on” is used to indicate a particular day or date. In this case, the speaker is not saying that they have not ridden a bike on a particular day or date, but rather that they have not ridden a bike since a specific point in time.
The word “none of the above” is not a correct answer to the question.