Final answer:
Intelligence agencies are the primary source for receiving indications and warnings of terrorist activities against Navy installations, due to their specialized resources and capabilities in threat analysis.
Step-by-step explanation:
The indications and warnings of terrorist activities against Navy installations are normally received from intelligence agencies. These agencies specialize in gathering and analyzing information related to potential threats, including terrorism, and are equipped with the necessary resources and capabilities to provide timely and credible warnings to military installations. While local news outlets, social media platforms, and base security personnel may also provide information regarding potential threats, the primary source for an indication or warning of terrorist activities is typically through formal intelligence channels.