To find the missing factor \( D \) in the equation
\[ -15y^4 = (D)(3y^2), \]
we can solve for \( D \) by isolating it. We start by dividing both sides of the equation by \( 3y^2 \):
\[ D = \frac{-15y^4}{3y^2}. \]
Now, simplify the right-hand side. First, divide the coefficients:
\[ -15 \div 3 = -5. \]
Next, simplify the \( y \) terms. We can use the property of exponents:
\[ y^4 \div y^2 = y^{4-2} = y^2. \]
Putting it all together, we have:
\[ D = -5y^2. \]
Thus, the missing factor is
\[ \boxed{-5y^2}. \]