C

- Hướng dẫn giải

Chọn C

$$\begin{gathered} \underset{t\to a}{\lim }\frac{t^4-a^4}{t-a} = \underset{t\to a}{\lim }\frac{(t^2-a^2).( t^2 +\hspace{0.17em}{a}^2)}{t-a} = \underset{t\to a}{\lim }\frac{(t- a).(t+ a) .( t^2 +\hspace{0.17em}{a}^2)}{t-a} \\ =\underset{t\to a}{\lim }\frac{\left(t-a\right)\left(t^3+a{t}^2+a^{2}t+a^3\right)}{t-a}= a^3 +\hspace{0.17em}a.a^2+\hspace{0.17em}{a}^2.a + a^3=4a^3 \end{gathered}$$