"One hundred Yuan"
Could 'bai yuan' also be understood as 100 yuan, or does it have to be specified as yi bai?
The "y" in pinyin "yi" is just a placeholder and is not pronounced, similar to the “w” in "wu". "Yi" in Standard Putonghua is always pronounced "ee" or [i] in IPA.
Y sounds ee, when the syllable starts with the sound ee, it's written yi in Piyin. So y is the same sound that i but at the start of a syllable.
Again only one choice! Put some effort into the multiple choice questions. Or ARE you wasting people's time on purpose?