'Should' is a modal verb and all modal verbs are followed by a base form of verb so the use of 'sorted' will be incorrect, it should be replaced with the verb 'sort'.
The correct sentence is- We should sort out the problem ourselves rather than depending on the third party.
Hence, option (c) is correct.
NOTE/-
Modal verb: A verb used with another verb to express an idea such as possibility that is not expressed by the main verb. The modal verbs in English are "can," "could," "may," "might," "must," "ought," "shall," "should," "will," and "would." They do not change their form (spelling) and they have no infinitive or participle (past/present). Example: I can speak Spanish. Here 'can' expresses the ability of the person to speak (main verb) Spanish. This ability cannot be expressed by the main verb 'speak'.
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