Final answer:
The number that best represents a positive charge of 1,861 is A. 1,861, since it precisely matches the magnitude and sign of the charge.
Step-by-step explanation:
The number that best represents a positive charge of 1,861 is A. 1,861. Since we are looking for a positive charge, we can immediately eliminate the options with a negative sign, which are options B and C. Between the remaining options A and D, option A correctly matches the magnitude of the positive charge without any alterations, making it the best representation of a positive charge of 1,861.