The length of arc required in welding depends on various factors. Let's discuss each of these factors in detail:

a) The kind of electrode used, its coating, and its diameter:
- Different types of electrodes are used for different welding processes. For example, in shielded metal arc welding (SMAW), commonly known as stick welding, electrodes with different coatings are used. The coating on the electrode helps in providing protection from the atmosphere and also influences the characteristics of the arc.
- The diameter of the electrode also affects the length of the arc. A smaller diameter electrode requires a shorter arc length, while a larger diameter electrode requires a longer arc length.

b) The magnitude of current used:
- The magnitude of current used in welding plays a crucial role in determining the length of the arc. Higher current levels generally require a longer arc length.
- The length of the arc is directly proportional to the current. As the current increases, the arc length also increases. This is because higher current levels generate more heat, and a longer arc is needed to dissipate the heat properly.

c) The position of welding:
- The position of welding, whether it is flat, horizontal, vertical, or overhead, affects the length of the arc.
- In vertical and overhead welding positions, a shorter arc length is generally preferred to prevent excessive spatter and maintain control over the molten metal.
- On the other hand, in horizontal and flat positions, a slightly longer arc length may be required to ensure proper penetration and fusion.

Considering all the factors mentioned above, the correct answer to the question is option 'D' - all of the above. The length of arc required in welding depends on the kind of electrode used, its coating, and its diameter, the magnitude of current used, as well as the position of welding. All these factors need to be taken into consideration to determine the appropriate length of the arc for a particular welding process.