I'd begin by determining how much salt, pepper and sugar are required.
5/8 + 1/4 = 7/8, so the salt, pepper and sugar comprise 1/8 of the mixture.
Since each spice is equally represented, we can say that salt, pepper and sugar EACH comprise 1/24 of the mixture.

At this point, let's assume that the regular recipe requires 24 parts.
The recipe calls for 15 parts olive oil (5/8 = 15/24), 6 parts vinegar (1/4 = 6/24), 1 part salt, 1 part pepper, and 1 parts sugar.

Here's what got added to the botched dressing:
15 parts olive oil
12 parts vinegar (double the recipe)
1 part salt
1 part pepper

There are 29 parts in total and there are 15 parts olive oil.
So, the dressing 15/29 olive oil. The answer is A