A certain bakery ran a promotion code: a customer can buy x

Responding to a pm:

The wording of the question is such that it is normal to read it twice to understand it well.

Promotion Code: a customer can buy x donuts for the regular price of $15 total and get 3 donuts free

This tells us two things:
The regular price of x donuts is $15. So regular price per donut is 15/x
In the promotion, you get 3 donuts free with x donuts. So promotion price per donut is 15/(x+3)

For a dozen, the promotion price is $2 less. So per donut, the promotion price is $2/12 less.

15/x - 2/12 = 15/(x+3)

Now try to plug in values of x from the options.
The equation holds if x = 15

If you face problems making equations in word problems, the issue is practice. You might struggle with 50 advanced word problem questions but then, when you go on to question no. 51, the equation will be more intuitive. Also, don't give up too soon while trying to solve a question (not talking about the actual or simulated tests here). Put in a good 10-15 mins to figure it out on your own before you finally give up and check out the solution. If you are able to solve it on your own, it will give you insight you will never forget.