Correct Answer: Strenuous

Solution : The correct choice is the first option.

Vigorous: This word describes something characterised by strength, energy, and robustness.

Strenuous: This is the correct synonym for vigorous . It means requiring or showing great effort, energy, or activity. Both vigorous and strenuous convey a sense of intensity and activity.

The meanings of the other options are as follows:

Delicate: It suggests fragility, sensitivity, or finesse.

Torpid: It means mentally or physically inactive, lethargic, or sluggish.

Guarded: It means cautious, careful, or reserved in speech or behaviour.

In summary, the closest synonym for vigorous is strength, as they both convey a sense of intense activity and effort.