**Core Concept**
The focal length of a high-power objective lens in a microscope determines its magnification and resolution capabilities. The objective lens is a critical component of a microscope, responsible for collecting and focusing light onto the specimen, thereby producing an enlarged image.

**Why the Correct Answer is Right**
The correct answer is 0.17 mm, which corresponds to a high-power objective lens. This lens has a short focal length, allowing it to produce high magnification (typically 40x to 100x) and a narrow field of view. The short focal length also enables the lens to collect light efficiently, resulting in a clear and detailed image of the specimen.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not correspond to the typical focal length of a high-power objective lens.

**Option B:** This option is incorrect because it is longer than the typical focal length of a high-power objective lens.

**Option C:** This option is incorrect because it is longer than the typical focal length of a high-power objective lens.

**Clinical Pearl / High-Yield Fact**
When using a microscope, it is essential to select the correct objective lens based on the specimen's size and the desired level of magnification. A high-power objective lens is ideal for examining small specimens or details at high magnification.

**Correct Answer: D. 0.17 mm**