The correct answer is: A. A rectangular pulse of duration T.
The autocorrelation function (ACF) of a signal is a measure of how similar the signal is to itself at different time delays. The ACF of a rectangular pulse of duration $T$ is a rectangular pulse of duration $T$, centered at the origin. This is because the ACF measures the correlation between the signal and itself, and a rectangular pulse is perfectly correlated with itself at all time delays.
The other options are incorrect because they do not represent the ACF of a rectangular pulse of duration $T$. Option B is a rectangular pulse of duration $2T$, which is not the ACF of a rectangular pulse of duration $T$. Option C is a triangular pulse of duration $T$, which is also not the ACF of a rectangular pulse of duration $T$. Option D is a triangular pulse of duration $2T$, which is also not the ACF of a rectangular pulse of duration $T$.