Step 1: Recall the 2024 Bharat Ratna announcement.
In early 2024, the Government of India announced Bharat Ratna for several distinguished individuals in recognition of their lifelong contributions to the nation.
Step 2: Identify the former PM awarded posthumously.
Charan Singh, former Prime Minister of India (1979-80) and a champion of farmers' rights, was conferred the Bharat Ratna posthumously in 2024.
Step 3: Distinguish other former PMs.
I.K. Gujral and Manmohan Singh did not receive this honour in 2024; Rajiv Gandhi received it in 1991.
\[ \boxed{\text{Charan Singh}} \]