To find the Highest Common Factor (HCF) of two numbers, we can use the Euclidean algorithm. This algorithm involves dividing the larger number by the smaller number and then taking the remainder. The divisor becomes the new dividend, and the remainder becomes the new divisor. This process is repeated until the remainder becomes zero. The last non-zero remainder is the HCF of the two numbers.

In this case, we need to find the HCF of 2511 and 3402.

Step 1: Start by dividing the larger number, 3402, by the smaller number, 2511.

3402 ÷ 2511 = 1 remainder 891

Step 2: Since the remainder is non-zero, we continue the process by dividing 2511 by the remainder, 891.

2511 ÷ 891 = 2 remainder 729

Step 3: Again, we divide the previous remainder, 891, by the new remainder, 729.

891 ÷ 729 = 1 remainder 162

Step 4: Next, we divide 729 by 162.

729 ÷ 162 = 4 remainder 81

Step 5: Finally, we divide 162 by 81.

162 ÷ 81 = 2 remainder 0

Since the remainder has become zero, we stop the process.

Step 6: The last non-zero remainder was 81. Therefore, the HCF of 2511 and 3402 is 81.

Hence, the correct answer is option 'D'.