Correct Answer: To watch over closely.

Solution : The most appropriate option is the third option.

People use the idiom "to keep an eye on" to convey the idea of watching over closely or monitoring someone or something attentively. It implies a sense of vigilance or supervision.

The meanings of the other options are as follows:

To be able to understand everything: This option is more about comprehension or understanding and doesn't convey the idea of active monitoring or keeping watch.

To see someone in a bad light: This option refers to forming a negative opinion about someone, which is not synonymous with keeping an eye on them.

To keep distance from: This option suggests maintaining physical distance but doesn't capture the essence of actively watching or monitoring someone.

Therefore, the correct answer is to watch over closely.