Option 1, Sagar Rakshak Bal, is not the correct answer. The term "Rakshak" usually translates to "protector" in English, but it does not specifically refer to protecting naval assets.

Option 2, Sagar Suraksha Bal, is not the correct answer. "Suraksha" means "security" or "safety," but it does not specifically indicate protection of naval assets.

Option 3, Sagar Prahari Bal, is the correct answer. "Prahari" means "guard" or "watchman." Therefore, Sagar Prahari Bal refers to the unit that is specifically tasked with protecting the naval assets.

Option 4, Sagar Nigrani Bal, does not accurately describe the unit responsible for protecting naval assets. "Nigrani" means "surveillance" or "monitoring," which is related to observing or keeping a watch on something, but it does not necessarily involve protection or guarding.

In conclusion, the correct answer is option 3, Sagar Prahari Bal, as it indicates the unit responsible for protecting naval assets.