What is the complement of the language accepted by the NFA shown below? Assume $\Sigma = \{a\}$ and $\varepsilon$ is the empty string.

Let us analyze the language accepted by the given NFA:

1. The start state (leftmost) is not an accepting state.

2. Reading an '$a$' takes the machine to the middle state, which is the only accepting state.

3. From the accepting state, an $\varepsilon$-transition goes to the rightmost state, and another $\varepsilon$-transition goes from the rightmost state back to the start state.

4. From the start state, another '$a$' can be consumed to reach the accepting state again.

Hence, any string with one or more '$a$'s can be accepted, i.e., $L = \{a, aa, aaa, \dots\} = a^+$.

The empty string $\varepsilon$ is not accepted because the start state is not an accepting state and there is no $\varepsilon$-path from the start state to the accepting state.

Since the alphabet is $\Sigma = \{a\}$, the universal language is $\Sigma^* = a^* = \{\varepsilon, a, aa, aaa, \dots\}$.

The complement of the language is:

Thus, the complement is $\{\varepsilon\}$, which corresponds to option (B).