The correct answer is A. Linear.
The structure of (HPO3)3 is linear because the central phosphorus atom is bonded to three oxygen atoms in a trigonal planar arrangement. The three oxygen atoms are also bonded to each other, forming a ring. This ring structure is what gives (HPO3)3 its linear shape.
Option B is incorrect because a tetrahedral structure has four atoms bonded to a central atom. In (HPO3)3, the central phosphorus atom is only bonded to three oxygen atoms.
Option C is incorrect because a cyclic structure has atoms bonded to each other in a ring. In (HPO3)3, the central phosphorus atom is bonded to three oxygen atoms, and the three oxygen atoms are also bonded to each other. This does not form a ring structure.
Option D is incorrect because an octahedral structure has six atoms bonded to a central atom. In (HPO3)3, the central phosphorus atom is only bonded to three oxygen atoms.