- For condition (i), the restriction is put on $a ; N<4000$ if $a<4$, and $N \geq 6$ if $a \geq 6$. Therefore, $a=4,5$.
- For condition (ii), the restriction is put on $d$; it must be a multiple of 5 . Therefore, $d=0,5$.
- For condition (iii), the restriction is put on $b$ and $c$. The possible ordered pairs of $b$ and $c$ are $(3,4),(3,5),(3,6),(4,5),(4,6)$, and $(5,6)$, and there are 6 of them. Alternatively, we are picking from the four digits $3,4,5,6$, and for every combination of two, there is exactly one way to arrange them in increasing order, so we have $\binom{4}{2}=6$ choices for $b$ and $c$ when we consider them together.
Multiplying the possibilities for each restriction, $2 \cdot 2 \cdot 6=24 \Rightarrow(\mathrm{C})$.
