**Core Concept**
Base substitution refers to the replacement of one nucleotide base with another in DNA, which can result in changes to the amino acid sequence of a protein. This type of mutation can be classified based on the nature of the change and its impact on the protein sequence.

**Why the Correct Answer is Right**
The given base substitution, GAC (Asp) to GAG (Glu), is an example of a point mutation. It is a type of non-silent mutation, as it changes the amino acid from aspartic acid (Asp) to glutamic acid (Glu). This change affects the protein's structure and function. The correct answer, option C, includes both "a" (point mutation) and "d" (non-conserved mutation), as the substitution of GAC to GAG is not conserved across different species.

**Why Each Wrong Option is Incorrect**
**Option A:** Point mutation is correct, but the question also asks for another type of mutation. Option A is missing the second type of mutation.
**Option B:** Silent mutation is incorrect because the substitution of GAC to GAG is not silent, as it changes the amino acid from aspartic acid to glutamic acid.
**Option C:** Non-sense mutation is incorrect because the substitution of GAC to GAG does not create a premature stop codon, which is characteristic of a nonsense mutation.
**Option D:** Non-conserved mutation is correct, but the question also asks for another type of mutation. Option D is missing the first type of mutation.

**Clinical Pearl / High-Yield Fact**
When considering point mutations, remember that a change from a codon that codes for one amino acid to another can result in a change to the protein's structure and function. This can have significant effects on the protein's activity and overall cellular function.

**Correct Answer:**
✓ Correct Answer: C. ad. Point mutation and non-conserved mutation.