Explanation:

- The thermometer is an instrument for measuring temperature.

- The ………. form consists of a ………. tube with a fine ………..

- One end of the ………… is blown to form …………… bulb and the other is closed.

Options:

1. Seen: Suggests something that is visible or observed, not applicable for a form style.

2. Unseen: Indicates something invisible, hence unrelated to a typical form style.

3. Heard: Relates to auditory perception, unsuitable for describing a form style.

4. Common: This option is the most appropriate, referring to the traditional or usual type of thermometer.

The correct answer is option 4. "common" is the most fitting word to describe the form of a thermometer, emphasizing its widespread and standard appearance.