Concept:The reflexive pronoun must agree with the subject it refers back to in the sentence.
Explanation:The sentence is: "They told her that we would solve the problem ...."
The subject of the verb "would solve" is "we".
Therefore, the reflexive pronoun should match "we".
The correct reflexive pronoun for "we" is "ourselves".
The other options refer to different subjects: herself (she), himself (he), themselves (they).
So the completed sentence is: "They told her that we would solve the problem ourselves."
Answer:D. ourselves