The correct answer is option (E):
Salubrious
The correct answer is "Significant".
The word "tenuous" means flimsy, weak, or insubstantial. It suggests something that is easily broken or easily doubted. The best antonym from the choices is "Significant," because significant means important, meaningful, or substantial. This directly opposes the idea of something being weak or insubstantial, making it the most appropriate opposite in meaning.
Let's look at the other options:
* "Ponderous" means heavy or clumsy, and is not a direct antonym for tenuous.
* "Fragile" is a synonym of tenuous, meaning easily broken.
* "Lengthy" refers to something that is long in duration or extent, and doesn't relate to the strength or substance of something.
* "Salubrious" means healthy, which is also not the opposite of tenuous.
Therefore, "Significant" is the only word that offers a direct and contrasting meaning to "tenuous," making it the correct antonym.