The correct statement for the manager to present to the class is option C: "When financing a purchase of securities, financial institutions often sell the repo to avoid putting up full purchase price for the securities." This is because selling a repo, also known as entering into a repurchase agreement, allows the financial institution to sell securities and agree to repurchase them at a later date, thereby not having to pay the full purchase price upfront. This is a common practice for managing liquidity and cash flow.

Option A is incorrect because to short a bond, a trader would typically enter into a reverse repurchase agreement, not a repo, to borrow the bond.

Option B is incorrect because margin calls are indeed common in repo transactions. Haircuts on collateral are charged to those who lend collateral, but margin calls are used to ensure that the value of the collateral remains sufficient to cover the loan.

Option D is incorrect because money market mutual funds usually enter into reverse repo agreements, not repos, to invest in short-term instruments. This allows them to lend their funds to other parties in exchange for collateral and earn interest.