Dose response curves of salbutamol for bronchodilation and tachycardia are widely separated on the dose axis. This information suggests that salbutamol is:

Dose response curve of any drug: The left sided curve is representing the bronchodilation and the right sided curve is representing the tachycardia effect of salbutamol. If the distance between the two curves increases, the drug will produce only the desired effect at low dose, side effects will appear only at very high dose. The drug will be more safer. More the distance between left and right curves, more selective the drug is. So the answer is'd'.