Must be true. In order to be preferable to semiconductors for use in digital circuits, vacuum tubes would to be comparable to semiconductors in all significant respects. Because vacuum tubes are not comparable in terms of current capacity, they are not now preferable.

Could be false. We don’t know if vacuum tubes are comparable to semiconductors in all other significant respects, so we don’t know if they would be preferable even with increased current capacity. Even if they were preferable, that doesn’t necessarily mean they would be used.

Could be false. The stimulus doesn’t tell us whether or not vacuum tubes are comparable to semiconductors in any way other than current capacity and heat resistance—maybe they’re deficient in other respects!

Could be false. The stimulus tells us absolutely nothing about whether or not semiconductors are preferable to vacuum tubes in applications other than digital circuits.

Could be false. Maybe vacuum tubes have other advantages besides heat resistance! The stimulus does not rule out this possibility.