After reading Ron's explanation, I am confused now. Kindly help me to understand parallelism between "being and causing"
These are two "-ing" words that could be parallel. For example: 'At the party Ron was being silly and causing all kinds of trouble.' When you're analyzing parallel structures, I'd encourage you to take note of the parallel marker word(s) (here: 'and'), the parallel elements (here: 'being' and 'causing') and the root phrase (here: 'At the party Ron was...').
I found 2 splits in option A :
1. Found X as being
2. missing parallelism between "has found ..... and causing..."
I'm not quite sure what you mean. We usually talk about "splits" as differences between answer choices. So, logically, a single answer cannot have splits on its own.
Perhaps you mean 'problems' with A. I agree with you about 1. As Ron discussed above, it's unidiomatic to say 'found X as being'. As for 2, I disagree; I think you've analyzed the parallel structure here incorrectly. The parallelism in A is between 'being' and 'causing'.
Remember that when we analyse a parallel structure with 'and', we often have more than one way of cutting up the sentence since 'and' doesn't tell us where the root phrase ends and the first element begins. It's important to apply the 'principle of charity' i.e. to take the most logical interpretation of the sentence to give it the best chance of being correct. If we do that and the sentence
still doesn't make sense, then we know that there's a problem with it. If we followed your interpretation, then the sentence would be saying: A recent study has found amoxicillin....and causing fewer side effects. How could the recent study cause side effects? This really doesn't make sense.