Which among the following writers remarked that “I know of no more encouraging fact than the unquestionable ability of man to elevate his life by a conscious endeavour”?

The correct option is Henry David Thoreau.

Detailed Explanation:
The quote, "I know of no more encouraging fact than the unquestionable ability of man to elevate his life by a conscious endeavour," is one of the most celebrated remarks by the American author, philosopher, and naturalist Henry David Thoreau. It is found in his masterpiece Walden (1854), specifically in the chapter titled "Where I Lived, and What I Lived For."

In the context of Walden, this quote encapsulates the core philosophy of Transcendentalism, an intellectual movement of the 19th century in which Thoreau was a leading figure. Transcendentalism emphasized individualism, self-reliance, and the inherent goodness of people and nature. Thoreau believed that individuals should not merely exist or conform to societal expectations, but should actively and deliberately construct their own lives. By "conscious endeavour," Thoreau meant living mindfully and making deliberate choices to align one's daily life with higher spiritual and moral ideals.

While Ralph Waldo Emerson was Thoreau's close friend and mentor who shared similar Transcendentalist beliefs, and writers like Henry James and Emily Dickinson contributed uniquely to American literature, this specific quote belongs strictly to Thoreau's reflections on his experiment of simple living at Walden Pond.