Geometrical isomers are a type of stereoisomers that occur due to restricted rotation around a double bond or a ring structure. They have the same molecular formula and connectivity but differ in the spatial arrangement of atoms in the molecule. Geometrical isomerism is commonly observed in compounds with double bonds, such as alkenes and cyclic compounds.

a) Diastereomers or Enantiomers:
The correct answer is option A - Geometrical isomers can be diastereomers or enantiomers. Let's understand each of these terms:

1. Diastereomers: Diastereomers are stereoisomers that have different spatial arrangements around one or more stereocenters in a molecule. In the case of geometrical isomerism, the double bond or ring structure acts as a stereocenter. Diastereomers have different physical and chemical properties and can be separated using conventional separation techniques. They have different names and exhibit different melting points, boiling points, and optical activities.

2. Enantiomers: Enantiomers are a specific type of stereoisomers that are non-superimposable mirror images of each other. They have the same connectivity and molecular formula but differ in the spatial arrangement of atoms in three-dimensional space. Enantiomers have identical physical and chemical properties except for their interaction with polarized light. They rotate the plane of polarized light in equal magnitudes but in opposite directions. Enantiomers are named using R and S configuration or D and L nomenclature.

b) Structural isomers:
Structural isomers are a type of isomers that have the same molecular formula but differ in the connectivity of atoms within the molecule. In structural isomerism, the atoms are connected in different ways, leading to different chemical and physical properties. Geometrical isomers are not classified as structural isomers because they have the same connectivity of atoms.

c) Interconvertible at room temperature:
Geometrical isomers are not interconvertible at room temperature because they require the breaking and reformation of chemical bonds for interconversion. Interconversion between geometrical isomers typically requires the application of heat or the use of a catalyst.

In conclusion, geometrical isomers can be diastereomers or enantiomers, but they are not classified as structural isomers. They are not interconvertible at room temperature.