To use dollar-cost averaging, an investor must invest:

in securities with high average yields.
in dollars rather than currencies from outside the U.S. or Canada.
a variable amount of money at regular intervals.
the same amount of money in stocks regularly.

I think its D, but not sure, please help!

You're correct.

To use dollar-cost averaging, an investor must invest a variable amount of money at regular intervals. This investment strategy involves regularly investing a fixed dollar amount into a particular security, regardless of its price. The key concept is to invest the same dollar amount consistently, regardless of whether the security's price is high or low. By doing so, the investor buys more shares when the price is low and fewer shares when the price is high.

This strategy helps to average out the cost per share over time, which can mitigate the risks associated with market fluctuations. Dollar-cost averaging is often recommended as a way to smooth out the impact of short-term price volatility and to take advantage of the long-term growth potential of the market.

It's important to note that dollar-cost averaging can be applied to various securities such as stocks, bonds, mutual funds, and exchange-traded funds (ETFs). The choice of securities will depend on the investor's investment goals, risk tolerance, and investment strategy.