Explanation

(a)
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This compound contains five carbons atoms, so it is a pentose. Its first carbon contains $–H\ and - OH\$group. This suggests that it is an aldose (i.e., contains aldehyde group). Since, its structure is similar to furan (a heterocyclic conpound), so it has furanose structure.
Hence, this compound is a pentose, aldose and have furanose structure.