Answer
$k$
Work Step by Step
We know
$$\lim\limits_{\theta \to 0} \dfrac{\mathrm{sin}\left(\theta \right)}{\theta }=1.$$
Thus for any nonzero real number $a$, it follows that
$$\underset{\theta \to 0}{\lim}\dfrac{\mathrm{sin}\left(a\theta \right)}{a\theta }=1.$$
We will use this fact to compute the limit below. So to see that this is true, we let $x=a\theta.$ Then since $x \to 0$ as $\theta \to 0$, we have $$\underset{\theta \to 0}{\lim}\dfrac{\mathrm{sin}\left(a\theta \right)}{a\theta }=\underset{x\to 0}{\lim}\dfrac{\mathrm{sin}\left(x\right)}{x}=1.$$
Now, we want to find $\underset{t\to 0}{\lim}\dfrac{\mathrm{sin}\left(kt\right)}{t}$, where $k$ is a constant.
We first suppose $k=0$. Then
$$\underset{t\to 0}{\lim}\dfrac{\mathrm{sin}\left(kt\right)}{t}=\underset{t\to 0}{\lim}\dfrac{\mathrm{sin}\left(0\right)}{t}=\underset{t\to 0}{\lim}0=0=k.$$
Now suppose $k\neq 0$. Then
$$\underset{t\to 0}{\lim}\dfrac{\mathrm{sin}\left(kt\right)}{t}=\underset{t\to 0}{\lim}\dfrac{k\mathrm{sin}\left(kt\right)}{kt}=k\left(\underset{t\to 0}{\lim}\dfrac{\mathrm{sin}\left(kt\right)}{kt}\right)=k\left(1\right)=k.$$
Thus for any constant $k$, $\underset{t\to 0}{\lim}\dfrac{\mathrm{sin}\left(kt\right)}{t}=k$.
