(a) The given circuit is equivalent to a parallel combination of two identical capacitors.
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Hence, equivalent capacitance between points $A\ and\ B$ is
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ C = \frac{\varepsilon_{0}A}{d} + \frac{\varepsilon_{0}A}{d} = \frac{2\varepsilon_{0}A}{d}$