Final answer:
The greatest common factor (GCF) of x⁴ and x⁵ is x⁴.
Step-by-step explanation:
The greatest common factor (GCF) of x⁴ and x⁵ can be found by identifying the highest power of x that divides both terms. Since x⁴ is a factor of x⁵ (x⁵ = x⁴ * x), the GCF is x⁴.
Therefore, the correct answer is a. x⁴.