Assertion: Interferons are effective against viruses.
Reason: Proteins which can be synthesised only be genetic engineering are effective against viruses.

Answer

**Step-by-Step Solution:**

1. **Understanding the Assertion:**
- The assertion states that "Interferons are effective against viruses." This is true. Interferons are a group of signaling proteins produced by host cells in response to the presence of pathogens, such as viruses. They play a crucial role in the immune response by interfering with viral replication and enhancing the immune system's ability to fight off infections.

2. **Understanding the Reason:**
- The reason provided is "Proteins which can be synthesized only by genetic engineering are effective against viruses." This statement is misleading. While some proteins that are engineered can be effective against viruses (like certain monoclonal antibodies), not all proteins effective against viruses require genetic engineering. Interferons, for instance, can be naturally produced by the body without genetic engineering.

3. **Evaluating the Truth of Assertion and Reason:**
- The assertion is true (interferons are indeed effective against viruses).
- The reason is false (not all effective antiviral proteins are synthesized only through genetic engineering).

4. **Conclusion:**
- Since the assertion is true and the reason is false, the correct answer is that the assertion is true but the reason is false. This corresponds to option C.