The correct answer is A. Deflect downward.
When a simply supported beam has reinforcement only at the bottom, the concrete in the top of the beam will shrink more than the concrete in the bottom of the beam. This will cause the top of the beam to deflect downward.
The reinforcement in the bottom of the beam will resist the downward deflection, but it will not be able to prevent it completely. The amount of deflection will depend on the amount of shrinkage, the strength of the concrete, and the amount of reinforcement.
If the reinforcement is not strong enough, the beam may crack. If the reinforcement is too strong, the beam may be over-designed and will be more expensive than necessary.
It is important to design beams with the correct amount of reinforcement to prevent cracking and to minimize the cost of the beam.