Which feature of the liquid in glass thermometer increases its sensitivity?

To increase the sensitivity of a liquid-in-glass thermometer, two main features are important:

1. A large bulb to contain a significant volume of liquid, so that even a small temperature change results in a noticeable expansion of the liquid.
2. A narrow capillary tube (the stem where the liquid rises). This ensures that for a given volume expansion of the liquid, the column rises a greater distance, making the thermometer more responsive and easier to read small temperature changes.

Let's evaluate the given options:

• A. Narrow capillary ball: A narrow capillary tube increases sensitivity because a small expansion of the liquid causes a larger rise in the column. The term "ball" here is likely referring to the bulb or is a slight misphrasing, but "narrow capillary" is the key part that increases sensitivity.
• B. Wider capillary ball: A wider capillary tube would decrease sensitivity, as the liquid column would rise less for the same volume expansion.
• C. Small bulb: A small bulb contains less liquid, so for a given temperature change, the total volume expansion is smaller, which decreases sensitivity. A large bulb increases sensitivity.
• D. Wide range: A wide range refers to the span of temperatures the thermometer can measure, not its sensitivity to small temperature changes.

Therefore, a narrow capillary is the feature that increases sensitivity.

The final answer is $\boxed{\text{A}}$