The issue here isn't whether parts are plural or not. The word 'parts' is obviously plural, isn't it? One part, two parts. If you can count something .. one, two, three .. and add an 's', it is clearly a plural word. The issue is about the use of the phrase 'There's'. In standard grammar, we use 'There's' for a singular noun, and 'There are' for a plural noun. The correct sentence should therefore be 'There are two parts to it.'. Note, by the way, that we don't usually write 'There're'. It isn't a standard contraction. But what about the native speaker who wrote 'There's two parts to it.' Is it completely wrong? Was it a careless mistake? Is it bad English? Is the writer lazy or badly educated? The answer to all of these questions is 'no'. It's simply a colloquial form. When speaking informally, I sometimes use it myself. It doesn't actually sound wrong to most native speakers.

B. There're two parts to it- correct Yes "parts" is plural.

B is correct. I would call A 'poor English'. Remember that natives aren't gods! They can make mistake. They might be wrong. They might not know it. They might not be well educated. They might be tired or absent-minded. ...so you were right to ask here and clarify it This is the right way to ask your question: Is 'parts' plural?

B is correct, however it's common in colloquial speech for native speakers to use "there's" in both singular and plural. BTW, your question should be "is 'parts' plural" because it's just one word.