Almost a million

Answer: One million minus 4

Dividing by 6

$$\begin{array}{rl}166666& \text{ remainder 4}\\ 6\overline{)1000000}& \end{array}$$ So $4$ less than $1000000$ is a multiple of $6$

Looking for multiples of 6

In order to be a multiple of 6, a number must be both even and a multiple of 3. Of the numbers given, only (b) 999 998 and (d) 999 996 are even.

For a number to be a multiple of 3, the sum of its digits must also be a multiple of 3. From this, we see that, of these two, only (d) 999 996 is a multiple of 3. Hence (d) is the only multiple of 6 here.