Explanation:
In an \(L - C - R\) circuit, phase angle is
defined as \(\cos \phi = \frac{R}{Z}\)
where, \(Z = \sqrt {{R^2} + {{\left( {{X_L} - {X_C}} \right)}^2}} \)
When circuit is resonant,
\(\,\,\,\,\,\,\,\,\,\,\,\,{X_L} = {X_C} \Rightarrow Z = R\)
So, \(\cos \phi = \frac{R}{R} = 1,\phi = \mathop 0\limits^ \circ \)