Step 1: Fiduciary duty of partners.
The Indian Partnership Act, 1932 (s.9) requires partners to act honestly and render true accounts and full information to any partner or his legal representative.
Step 2: Why (a) is incorrect.
(a) states the opposite — partners must disclose, not hide, material information.
Step 3: Other options.
(b) Correct — s.10 imposes indemnity obligation for fraud.
(c) Wrong in law — actually, a retired partner is liable for acts before retirement unless released by agreement (so statement (c) is factually incorrect in general law, but within question framing, (a) is most clearly “incorrect”).
(d) Correct — property to be used only for firm purposes (s.15).
\[
\boxed{(a)}
\]