Final answer:
The word that goes into both groups is 'doubt'. Words with 'b' spelled out are 'believe' and 'beauty', while words with 'b' pronounced as 'b' are 'debt', 'doubt', and 'subtle'.
Step-by-step explanation:
The word that goes into both groups is 'doubt'. In the first group, words with 'b' spelled out are 'believe' and 'beauty'. In the second group, words with 'b' pronounced as 'b' are 'debt', 'doubt', and 'subtle'. While the letter 'b' is spelled out in 'debt' and 'subtle', it is not pronounced as 'b' in those words. Lastly, when referring to a compound, a mixture, a metal alloy, an element, and a homogeneous mixture, it suggests a question from chemistry where students are likely being asked to classify substances into these categories based on their composition and properties.