**Core Concept:** Bacteriophage is a virus that infects bacteria, and its discovery is crucial to understanding the interaction between viruses and their host organisms, as well as the development of antibiotics and anti-infective therapies.

**Why the Correct Answer is Right:** The correct answer, Dmitri Ivanovsky, is the Russian virologist who discovered bacteriophages in 1928. He noticed that tobacco mosaic virus-infected tobacco plants had symptoms similar to bacterium-infected bacteria, which led him to hypothesize the existence of a virus that infects bacteria. Ivanovsky purified these "ghost particles" from infected bacteria, demonstrating their infectious nature and proving his hypothesis.

**Why Each Wrong Option is Incorrect:**

A. Felix d'Herelle (Option A) is incorrect because he is not the discoverer of bacteriophages. Instead, he discovered the bacteriophage's ability to kill bacteria and named them "bacteriophage" in 1915.

B. Frederick Twort (Option B) is incorrect because he independently discovered bacteriophages in 1915, but Ivanovsky provided the crucial evidence for their existence and demonstrated their infectious nature.

C. Both Ivanovsky (Option D) and Twort (Option B) are correct, but the question asks for the discoverer, which is Ivanovsky.

D. Frederick Twort (Option B) is incorrect because he also discovered bacteriophages in 1915, but Ivanovsky provided the crucial evidence for their existence and demonstrated their infectious nature.

**Clinical Pearl:** The discovery of bacteriophages has significant implications for medical science, as it led to the development of antibiotics. Bacteriophages can specifically infect and kill target bacteria, providing a potential alternative or adjunct to antibiotic therapy in combating antibiotic-resistant bacterial infections.