This word wasn't taught. How can we be expected to learn it before this point?
I would gladly explain, if only I knew what two ways of spelling are you talking about exactly.
I believe you meant "ciasteczko" and "ciasteczka", and if so, then the first one is singular and the second plural, indeed.
They might mean that there are 2 answers, ciasteczka and ciastka? I didn't know about the 2nd one.
Oh. Well, people who created this skill decided to go with the word "ciasteczko" for a cookie (which for me seems strange as it is diminutive, I'd definitely go with "ciastko"), but if they did, then I guess we should keep the starred answers this way. Therefore, "ciastka" shouldn't probably be a starred answer, as it wasn't taught... it will just be an accepted answer now.
I think that is the point, you learn through context and exposure, not classroom repetition
Well, the first sound is the most difficult. It is a palatalized c (English tz/ts), something not present in English. The rest simply goes: a-stech-ka. If you cannot pronounce this ć/ci sound, the closest will be ch: cha-stech-ka. Better this than nothing. And with time, you will learn to perceive and pronounce the palatalized sounds better.
Both mean the same. But in fact, "ciasteczka" is a diminutive of "ciastka".
The team that has created this course decided to teach "ciasteczka", which the current team considers to be a rather surprising decision, we would prefer to teach "ciastka".