
- ✓But $-3-$ enoic acid
- BBut $-1-$ enoic acid
- CPent $-4-$ enoic acid
- DProp $-2-$ enoic acid

So, the correct name is: but -$3$ - enoic acid.
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$(1)$ $\begin{matrix}
C{{H}_{3}} \\
|\,\,\,\,\,\, \\
ZNH\,C\,HC{{O}_{2}}H \\
\end{matrix}$ $(2)$ $\begin{matrix}
C{{H}_{3}}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, \\
|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, \\
{{H}_{2}}N\,C\,HC{{O}_{2}}C{{H}_{2}}{{C}_{6}}{{H}_{5}} \\
\end{matrix}$
$(3)$ $\begin{matrix}
\,\,\,\,\,\,\,\,\,\,\,\,C{{H}_{3}}{{C}_{6}}{{H}_{5}} \\
|\,\,\,\,\, \\
ZNH\,C\,HC{{O}_{2}}H \\
\end{matrix}$ $(4)$ $\begin{matrix}
C{{H}_{2}}{{C}_{6}}{{H}_{5}}\,\,\,\,\, \\
|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\, \\
{{H}_{2}}N\,CHC{{O}_{2}}C{{H}_{2}}{{C}_{6}}{{H}_{5}} \\
\end{matrix}$