Step 1: Understand the historical context.
The Doctrine of Lapse was introduced by Lord Dalhousie in the mid-19th century. It allowed the British to annex any princely state if the ruler died without a natural heir. This policy caused discontent among Indian rulers, as it led to the annexation of several states like Satara, Jhansi, and others.
Step 2: Conclusion.
The correct reason for the discontent among rulers was Lord Dalhousie's adoption of the Doctrine of Lapse.
Final Answer:
\[
\boxed{ \; he \; adopted \; the \; 'Doctrine \; of \; Lapse.'}
\]