Iodometric and Iodimetric Titrations
Iodometric and iodimetric titrations are both analytical techniques used in chemistry to determine the concentration of a specific substance in a sample. These titrations rely on the redox reaction between iodine and the substance being analyzed.

Estimation of Oxidizing Agents
In iodometric titrations, iodine is generated by the reaction of an oxidizing agent with an excess of iodide ions in an acidic solution. The iodine produced is then titrated with a reducing agent, such as sodium thiosulfate, until the reaction is complete. The amount of oxidizing agent present in the sample can be determined by calculating the stoichiometry of the reaction.

Estimation of Reducing Agents
In iodimetric titrations, iodine is generated by the reaction of a reducing agent with an excess of iodate ions in an acidic solution. The iodine produced is then titrated with a standard solution of sodium thiosulfate until the reaction is complete. The amount of reducing agent present in the sample can be determined by calculating the stoichiometry of the reaction.

Therefore, both iodometric and iodimetric titrations are used for the estimation of oxidizing agents as well as reducing agents, making statement a) and b) correct.

H3PO3 as a Dibasic Acid
H3PO3, also known as phosphorous acid, is a diprotic acid which means it can donate two protons (H+) per molecule. Each proton donation corresponds to the dissociation of one hydrogen atom from the acid molecule. Therefore, statement c) is correct.

H2SO3 as a Reducing and Oxidizing Agent
H2SO3, also known as sulfurous acid, can act both as a reducing agent and an oxidizing agent depending on the reaction conditions. In certain reactions, H2SO3 can donate electrons and undergo oxidation, acting as a reducing agent. In other reactions, H2SO3 can accept electrons and undergo reduction, acting as an oxidizing agent. Therefore, statement d) is correct.

In conclusion, all the statements a), b), c), and d) are correct. Iodometric and iodimetric titrations are used for the estimation of both oxidizing and reducing agents. H3PO3 is a dibasic acid, capable of donating two protons. H2SO3 can act as both a reducing agent and an oxidizing agent depending on the reaction conditions.