The answer is 158 smooth seed-coat plants.
The ratio of the wrinkled seed-coat plants to the smooth seed-coat plants is 3:1. Thus, if we have 4 seeds, 3 of them is wrinkled seed-coat plants and 1 is smooth seed-coat plants. Represented as percentage there are 75% (3/4) wrinkled seed-coat plants and 25% wrinkled seed-coat plants.
If there are 633 plants, there will be, again, 25% wrinkled seed-coat plants.
Since 25% of 633 is 158.25, there are 158 smooth seed-coat plants.