The phrase “break through” in this context refers to a significant or dramatic overcoming of a perceived obstacle, typically denoting a discovery or achievement that leads to significant progress.

Therefore, the option that best conveys the meaning of the underlined portion in the sentence is “an outstanding success” .

The result of his experiment represents an outstanding success in medical science. This implies that the outcome of the experiment has led to a substantial advancement or a notable achievement in the field of medical science.

The other options do not fit as well in this context:

• “Catastrophe” (Option B) and “a colossal failure” (Option E) suggest a disastrous outcome, which contradicts the positive connotation of a “breakthrough”.
• “An end to such experiments” (Option C) could imply a conclusion, but it doesn’t necessarily denote success or progress.
• “Breaking point” (Option D) typically refers to a point of crisis or a turning point, not a breakthrough or significant progress.