Zerodha does not accept fixed deposits or bank guarantees as collateral for margins due to operational and reconciliation constraints. These instruments are also subject to maturity and need to be renewed regularly to continue getting margin benefits.
However, an alternative to providing Fixed deposits as margins is investing your capital in a liquid fund, which Zerodha accepts as collateral with a haircut of only 10%.
Moreover, you can invest in Government Securities (G-secs) and pledge them as collateral to get margins. You can find the list of approved instruments that can be pledged for margins and the applicable haircut percentage by visiting zerodha.com/approved-securities.