If cost is Rs. 18,000 and profit is $$33\frac{1}{3}\% $$ on cost the amount of profit is

The correct answer is $\boxed{\text{C}}$.

The profit is calculated by multiplying the cost by the profit percentage. In this case, the profit percentage is $\frac{100}{3} + 33\frac{1}{3} = \frac{100 + 3 \times 100}{3} = \frac{400}{3} = 133\frac{1}{3}\%$. Therefore, the profit is $18,000 \times \frac{133\frac{1}{3}}{100} = \boxed{\text{Rs. 6,000}}$.

Option A is incorrect because it is the profit if the profit percentage was 20%. Option B is incorrect because it is the profit if the profit percentage was 30%. Option D is incorrect because it is not a valid option.