The given question asks us to match the tributaries with their main rivers. Let`s analyze each option:

Option 1: This option states that all the pairs (a, b, and c) are correctly matched. There are three pairs, so we need to check if all three are correct. Based on the correct answer (option 4), we know that at least one pair is incorrect. Therefore, option 1 is incorrect.

Option 2: This option states that pairs a and b are correctly matched. Again, we need to refer to the correct answer. As option 4 states that only pair c is correct, we can infer that option 2 is also incorrect.

Option 3: This option states that pairs b and c are correctly matched. However, the correct answer (option 4) only mentions that pair c is correct. Therefore, option 3 is incorrect.

Option 4: This option states that pair c is the only correctly matched pair. As per the correct answer, this is indeed the correct option.

Therefore, the correct answer is option 4.