This image is a minimal pair, squared - what I call a box set. One person says one of the phrases. The other has to listen and say A, B, C or D. The minimal pairs in this instance involve /s/ and /z/ - these are a pair of related consonants, the first unvoiced and the second, voiced. Success depends on the student accurately distinguishing words like peace / peas and price / prize, which can be quite tricky! But there is another difference at work apart from the consonant voicing - namely the vowel before it. Vowel sounds before an unvoiced consonant are clipped and so sound shorter than the same vowel sound before a voiced consonant. Indeed, research suggests that, for native listeners, the vowel length is the more important cue in distinguishing such pairs. One might be tempted therefore to focus on the vowel and not the consonant in such cases. However, research also suggests that non-native listeners rely more on consonant voicing as a cue (Anne Cutler, 'Native Listening', MIT press 2012). So there are arguments in favour of using either distinction in ELT classrooms. Or both - belt and braces. For example, I often begin by focusing on the /s/ /z/ distinction, and if students are having difficulty with that, add in the vowel length distinction. Then students can decide which works best for them. (More on vowel length distinction here)
This box set is one of a collection included in PronPack 3: Pronunciation Pairworks (forthcoming).