Let's analyze the question and the choices step by step: **Question:** Einstein's postulate that concerns the speed of light is that: **Choices:** 1. Rulers and clocks cannot be used to measure the speed of light 2. Nothing can travel faster than the speed of light 3. All uniformly moving observers measure the same speed of light 4. The speed of light is defined as 3.00 E8 miles per hour --- ### Step-by-Step Solution: #### Step 1: Understand Einstein's Special Relativity Postulate Einstein's second postulate of special relativity states: > **"The speed of light in a vacuum is the same for all observers, regardless of the motion of the light source or observer."** #### Step 2: Analyze Each Option 1. **Rulers and clocks cannot be used to measure the speed of light** - Incorrect. Rulers and clocks **can** be (and are) used to measure speed, including the speed of light. 2. **Nothing can travel faster than the speed of light** - This is a consequence of relativity, but **not** Einstein's actual postulate. 3. **All uniformly moving observers measure the same speed of light** - This is **exactly** Einstein's postulate. 4. **The speed of light is defined as 3.00 E8 miles per hour** - Incorrect. The speed of light is approximately \(3.00 \times 10^8\) **meters per second** (not miles per hour). #### Step 3: Choose the Correct Answer - The correct answer is: **All uniformly moving observers measure the same speed of light** --- ### **Final Answer** > **All uniformly moving observers measure the same speed of light**