In how many different ways can the letters of the word RIDDLED be arranged?

Answer: Option A

The given word contains 7 letters of which D is taken 3 times.
∴ Required number of ways
$$\begin{array}{rl}& =\frac{7!}{3!}\\ & =\frac{7\times 6\times 5\times 4\times 3!}{3!}\\ & =\left(7\times 6\times 5\times 4\right)\\ & =840\end{array}$$