Pronunciation of -ies, like the last syllable of "accessories" and "bees" in BrE

Accessory noun BrE /əkˈsesəri/

What is the pronunciation of the last syllable of accessories?

1. 
   

   / r iː z /

   
   


2. 
   

   / r i z /

   
   



3. 
   

   / r ɪ z /

   
   

My guess is 3. / r ɪ z /

Bee noun BrE /biː/

What is the pronunciation of the last syllable of bees?

1. 
   

   / b iː z /

   
   


2. 
   

   / b i z /

   
   


3. 
   

   / b ɪ z /

   
   

My guess is 1. / b iː z /

Answer

This is partly a matter of variation between accents, and partly just an arbitrary matter of "choosing a transcription system."

The variation is in the final vowel of "accessory." As I describe in this answer, the vowel here can be identified with either the vowel of "kit" or the vowel of "fleece" depending on the accent. The pronunciation tutor Geoff Lindsey has a blog article, "The fallac[ɪj]of schwee," where he recommends that English language learners use the vowel of "fleece" for this sound even if you're trying to use a British accent.

The vowel in "bee" is always the same as the vowel in "fleece."

Transcription systems

There are many IPA transcription systems for English. As John Wells explains in the linked article, one type, the "qualitative scheme," does not use the length marker /ː/. In this scheme, the "fleece" vowel is transcribed /i/, and the "kit" vowel is transcribed /ɪ/. The last syllables of "accessories" and "bees" would be transcribed /riz/ and /biz/ or /rɪz/ and /biz/ depending on the accent of the speaker.

Another type, the "quantitative-qualitative scheme," uses the length marker to distinguish "quantity." In this scheme, the "fleece" vowel is transcribed /iː/, and the "kit" vowel is transcribed /ɪ/. The last syllables of "accessories" and "bees" would be transcribed /riːz/ and /biːz/ or /rɪz/ and /biːz/ depending on the accent of the speaker... at least, that's probably the best phonemic transcription (although apparently some phonologists, such as Dwight Bollinger, do treat "weak" vowels as distinct phonemes).

The complicated part is that even for phonologists who only recognize two vowel phonemes in this region, the use of the length marker allows us to cover the two accents with a single transcription using a kind of "shortcut": /i/, which is not used otherwise in the quantitative-qualitative scheme, can be defined as "/ɪ/ for some speakers, and /iː/ for others." This is called a "diaphonemic transcription." It is convenient in some ways, but it can also be confusing. This would give us the transcriptions /riz/ and /biːz/.

As an English language learner, you should avoid thinking of this /i/ as a third vowel that has to be distinguished in pronunciation from /ɪ/ and /iː/. It would be more accurate to think of it as not being distinguished from either of them, but that's still an unnecessary complication: as Geoff Lindsey says, it's simple and correct to just pronounce it exactly the same as /iː/.

Phonetic realization

Phonetically, Lindsey says the British English vowel /iː/ can vary between more-or-less monophthongal [i] and more diphthongized pronunciations like [ɪj].