This answer is unsupported. We don’t know what factors could cause the threshold to be lowered, if at all.

This answer is anti-supported. We know that past a threshold, even dramatic increases in the consumption of cholesterol and fat only result in a gradual increase of serum cholesterol. And the average North American diet is well past that threshold.

This answer is strongly supported. If a person is consuming half as much cholesterol and fat than the average North American, then that person is still consuming double the the amount of these substances compared to the threshold. And we know that additional consumption past the threshold only causes a gradual increase of serum cholesterol.

This answer is unsupported. We only know that consumption of cholesterol and fat is one of the most important factors affecting serum cholesterol. It's entirely possible that there are other factors that could affect serum cholesterol levels.

This answer is anti-supported. We know from the stimulus that, before a threshold, serum cholesterol levels are directly proportional to a person’s consumption of fat and cholesterol. So below the threshold, less consumption directly means less serum cholesterol.