Ion responsible to prevent polyspermy at the time of feilization in mammals is –

**Core Concept**
The process of preventing polyspermy, or the fertilization of an egg by multiple sperm, is crucial for proper development in mammals. This involves specific **ion channels** and **cell signaling pathways** that block additional sperm from penetrating the egg after the first sperm has entered.

**Why the Correct Answer is Right**
Although the specific ion is not mentioned, the correct ion responsible for preventing polyspermy is **calcium (Ca²⁺)**. When a sperm penetrates the egg, it triggers a **calcium wave** that leads to the **cortical reaction**, altering the egg's membrane to prevent other sperm from fusing with it. This mechanism ensures that only one sperm fertilizes the egg.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the correct ion provided, we cannot directly address each option. However, typically, options that are not calcium or related to its pathway would be incorrect because they do not participate in the polyspermy prevention mechanism.
**Option B:** Similarly, without specifics, any ion not directly involved in the **calcium signaling pathway** would be an incorrect choice.
**Option C:** And any option that does not lead to the **cortical reaction** or alter the egg's outer layer to prevent polyspermy would also be incorrect.
**Option D:** The same reasoning applies, as the correct mechanism involves **calcium ions** and their effects on the egg's membrane.

**Clinical Pearl / High-Yield Fact**
The prevention of polyspermy is a critical step in fertilization, ensuring the development of a healthy embryo. Understanding the role of **ions and cell signaling** in this process can provide insights into fertility issues and the development of certain reproductive technologies.

**Correct Answer:** D. Calcium