If you'd tell us what you did, we could tell you what you did wrong!

Here's what we know about ponies - 1/2 ponies make a HUGE mess, so we can only have whole ones. In other words, the total number of ponies must be an integer.

5/6 of the ponies have horseshoes, so we know that the number of ponies must be a multiple of 6: elminate (b) and (c).

1/2 of the ponies with horsehoes are icelandic. Well, if only 5 ponies had horseshoes, then we'd have 2.5 icelandish ponies - no one wants to see that! In order to avoid a non-integer result, we must have started with at least 12 ponies (5/6 of 12 would gives us 10 with horseshoes; 1/2 of 10 would give us 5 icelandic).

Choose (a) 12.