Imagine if the earth is not inclined towards the plane of its orbit, then A.…

Concept: The circle of illumination is the great circle that separates the sunlit half of the Earth from the dark half at any instant. Its orientation relative to the network of meridians and parallels, and whether the intensity or duration of sunlight a place receives changes through the year, both depend on whether Earth's rotational axis is tilted relative to the perpendicular of its orbital plane.

Application: If the axis had no such inclination, it would stand perpendicular to the orbital plane and the Sun's rays would always fall vertically over the equator. The circle of illumination would then always pass through both the North and South Poles, so it would permanently coincide with the lines of longitude (statement A). Because the Sun's position relative to the equator would never shift over the year, every location would receive sunlight at the same angle and for the same duration in every month, so a given region would show no seasonal variation (statement C).

Contrast: Checking the remaining pairings against this shows why they fail:

• A and D — the illumination circle would still align with the longitudes, but removing the tilt removes the changing sun-angle that produces seasonal swings, so seasonal variation would disappear rather than increase.

• B and C — the circle of illumination is always a great circle, so it cannot coincide with a line of latitude (other than the special case of the equator); the no-tilt condition does not change this.

• B and D — neither the latitude-alignment claim nor the increased-variation claim follows from removing the axial tilt, for the reasons given above.

Result: Only the pairing of the longitude-alignment claim with the no-seasonal-variation claim holds, so “A and C” is correct.