The correct answer is: A. lower stability limit.
A synchronous machine with a low value of short-circuit ratio has a lower stability limit. This means that the machine is more likely to become unstable if there is a sudden change in load or power factor.
The short-circuit ratio is a measure of the ability of a synchronous machine to withstand a short circuit. It is defined as the ratio of the machine’s short-circuit current to its rated current. A higher short-circuit ratio indicates that the machine is more able to withstand a short circuit.
The stability limit of a synchronous machine is the maximum load that the machine can carry without becoming unstable. The stability limit is lower for machines with a low short-circuit ratio because these machines are more likely to become unstable if there is a sudden change in load or power factor.
The other options are incorrect. A synchronous machine with a low value of short-circuit ratio does not necessarily have good speed regulation or good voltage regulation.