Probably Easy

So, we have 5 letters to be posted in 10 letter boxes. But, we do not need to follow any special/particular pattern or order in posting them.

So, the $1^{st}$ letter has 10 boxes to choose from, in order to be posted.

Similarly, the $2^{nd}$ letter also has 10 boxes to choose from(we don't need to omit the box we have already chosen, as 2 or more letters can be posted in the same letter box).

Similarly, for the $3^{rd}$ , $4^{th}$ and the $5^{th}$ letter, we have 10 choices for each.

Thus, the total number of choices for all 5 letters is

$\large10\times10\times10\times10\times10=\boxed{10^{5}}$ .

So, the $6^{th}$ option among the choices is the correct answer.