For a circuit with a square wave input, what would be the output?

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When a square wave is applied to a simple integrator, the output becomes a triangular waveform. An integrator sums the input voltage over time, so a constant high (or low) input produces a linear ramp up (or down) in voltage. When the input switches, the ramp reverses direction, yielding a repeating triangular shape. The exact steepness of the ramps depends on the RC time constant relative to the square wave period, but the overall form remains triangular.

So the correct choice is the one that shows a triangular waveform, which reflects the integrating action of the circuit. If the circuit were a differentiator, you’d expect spikes at the transitions instead, and if it were just a buffer or a low-pass filter with a very fast edge, you’d still skew toward maintaining or smoothing the shape rather than producing a triangle.