To solve this problem, we need to follow the given instructions and replace every even letter beginning from B with an odd number beginning with 3. We are then asked to find the third letter/number to the left of the tenth number/letter when counting from our right.

Let's break down the problem into steps:

Step 1: Replace even letters with odd numbers
Starting from B, we replace every even letter with an odd number beginning with 3. The sequence would look like this:
B, 3, D, 5, F, 7, H, 9, J, 11, K, 13, M, 15, ...

Step 2: Counting from the right
We are asked to count from the right to find the third letter/number. This means we need to count backwards from the end of the sequence we obtained in step 1.

Step 3: Find the tenth number/letter from the right
To find the tenth number/letter from the right, we count 10 elements from the right end of the sequence. Starting from M, we count back 10 elements:
M, 15, K, 13, J, 11, H, 9, G, 7

Step 4: Find the third letter/number from the left
After finding the tenth number/letter from the right, we need to identify the third letter/number from the left in this group of 10 elements.

The third letter/number from the left would be the letter/number at the 8th position in the group:
M, 15, K, 13, J, 11, H, 9, G, 7

Therefore, the third letter/number from the left is H.

Step 5: Determine the correct option
Now that we have found the third letter/number from the left, we need to determine which option matches our answer.

Option C is 21. Since H is not equal to 21, we can eliminate option C.

Options A and B are letters, but we have already determined that the answer is H. Therefore, we can eliminate options A and B.

This leaves us with option D, which is 23. Since H is also not equal to 23, we can eliminate option D.

Therefore, the correct answer is option C.