A mutual fund investor’s cost basis is generally the total amount invested in the position, adjusted for actions that add to or reduce the investor’s investment in the fund. Reinvested dividends change cost basis because the dividends are used to purchase additional shares, which increases the investor’s total invested amount. Even if the dividends are paid from the fund and immediately reinvested, the IRS treats them as distributed to the shareholder and then used to buy shares. As a result, reinvestment increases the number of shares owned and increases total basis by the amount reinvested. Therefore, D is correct.

Choice A is incorrect because transferring a position via ACATS does not change the investor’s economic investment; it is simply moving the account/position between firms. The cost basis should transfer as part of the record (subject to cost basis reporting rules), but the act of transfer itself does not change basis. Choice B is incorrect because NAV movement changes market value, not what the investor paid; unrealized appreciation does not change cost basis. Choice C is incorrect because a new portfolio manager may affect future performance, but it does not alter the historical amounts the investor contributed or reinvested; thus it does not change basis.

For SIE purposes, the key relationship is: cost basis changes when the investor adds money (new purchases, dividend/cap gains reinvestments) or when basis is adjusted due to specific corporate/fund actions (like return of capital distributions in some contexts). Reinvested dividends are a standard, frequently tested basis-changing event because they affect taxable reporting and future gain/loss calculations upon redemption.