Answer:
The answer is the fourth sentence: There is no more unhealthy being, no man less worthy of respect, than he who either really holds, or feigns to hold, an attitude of sneering disbelief toward all that is great and lofty, whether in achievement or in that noble effort which, even if it fails, comes to second achievement,
Explanation:
Roosevelt begins to address cynical people in the previous sentence, "Let the man of learning, the man of lettered leisure, beware of that queer and cheap temptation to pose to himself and to others as a cynic, as the man who has outgrown emotions and beliefs, the man to whom good and evil are as one." Here, however, is much more describing what a cynical is than really stating his opinion.
In the following sentence, however, he openly conveys his opinion. Cynical people, according to him, are the unhealthiest ones, the type that is least worth of respect. Roosevelt despises the man who would rather do nothing while criticizing those who do something, who try. He sees such an attitude as a mix of pride and fear that leads to accomplishing nothing in life. Being knowledgeable but actionless is of no use to society. Cynical people are therefore nothing but cowards, too afraid to risk anything, too conceited to admit it.