Given that Michelle has not been able to commit to a regular exercise regime, the best answer is a word that describes irregularity. "Sporadically"means to do something at inconsistent intervals. Therefore, "sporadically"is the word that best fits the sentence.

The word "natural" comes from roots related to birth. Something that is "natural" is inborn. Thus, if someone has a naturally happy attitude, that person is happy in an unreflective way. He or she just "is happy"—as though he or she was born that way. The word has taken on the meaning "genuinely" because such a natural disposition is not something that is faked.

An "instant" is a very short period of time. Whenever something happens in a very short time, it happens "instantaneously." This means that it happens almost immediately.

When something occurs "periodically," it occurs regularly in a given period of time. For instance, the periodic swinging of a pendulum is the time that it takes for it to swing back and forth. Likewise, the occurrence of a full moon occurs periodically—according to a fixed period of the moon's orbiting around the earth. By itself, the word "periodically" does not indicate whether or not something occurs frequently, quickly, or rarely. It merely indicates that something happens at regular intervals.

The word "occasionally" is an adverb that could be rewritten, on occasion or on a given occasion. The implication is that it describes something that happens sometimes but not always. It does not necessarily state that it occurs rarely. Likewise, it is not proper to use "occasionally" to indicate that you do something frequently. You wouldn't say, "I occasionally eat asparagus," but actually mean that you eat it every day of the year. To indicate that you eat it every day of the year, you would say, "I always eat asparagus." To say "occasionally" is to indicate that the activity being described is sporadic—now and then, on occasion.

Annually means every year or yearly.

"Seldom" is an adverb that means not happening often. The best choice is "rarely" because it means infrequently and is a synonym of "seldom." Something that seldom happens rarely happens.