Concept:
The degree of a differential equation is defined only when the equation is a polynomial in its derivatives after removing radicals and fractions involving derivatives.
ip
Step 1: Inspect the given differential equation.
The equation is:
\[
\frac{d^2y}{dx^2}+3\left(\frac{dy}{dx}\right)^2=x^2\log\left(\frac{d^2y}{dx^2}\right)
\]
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Step 2: Check whether it is polynomial in derivatives.
The highest derivative
\[
\frac{d^2y}{dx^2}
\]
appears inside a logarithm.
So the equation is not polynomial in derivatives.
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Step 3: Conclude about the degree.
Since the equation is not polynomial in derivatives, its degree is not defined.
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Hence, the correct answer is:
\[
\boxed{(D)\ \text{Not defined}}
\]