The standard specification of a grinding wheel is expressed in the form of a code that provides information about the type, size, and composition of the wheel. In this case, the code is A-46-M-6-V-21. Let's break down the code and understand its meaning:

1. Type: The first letter in the code represents the type of abrasive material used in the grinding wheel. In this case, the letter 'A' indicates that the wheel is made of aluminum oxide.

2. Grain Size: The second part of the code is a number that represents the mesh size of the abrasive grain. Mesh size refers to the number of openings per linear inch in the screen through which the grains are sifted during the manufacturing process. In this case, the number '46' indicates a medium grain size.

3. Grade: The next letter in the code represents the hardness or strength of the grinding wheel. The letter 'M' indicates a medium-hardness wheel.

4. Structure: The next number in the code represents the structure or spacing between the abrasive grains. In this case, the number '6' represents a dense structure.

5. Bond Type: The letter 'V' in the code represents the type of bond used to hold the abrasive grains together. In this case, the letter 'V' indicates a vitrified bond, which is a type of bond made from a mixture of clay and ceramic materials.

6. Manufacturer's Identity: The final part of the code is a number that represents the manufacturer's identity or specific product code. In this case, the number '21' is the manufacturer's identity.

From the given code A-46-M-6-V-21, we can conclude that the grinding wheel is made of aluminum oxide with a mesh size of 46. The medium hardness and dense structure make it suitable for general-purpose grinding applications. The vitrified bond provides good strength and durability to the wheel. It is important to note that the code may vary depending on different standards and manufacturers, but the general format and information remain similar.

Therefore, the correct answer is option 'C' - Aluminum oxide of mesh size 46.