Number of representative particles in 0.250 mol silver

A mole is just a specific number = 6.022 x 10^23 {actually this is approximately the number}. So one mole of a particular thing is 6.022 x 10^23 of those things. In a 0.250 mole silver = 0.250 (6.022 x 10^23) silver = 1.506 x 10^23 silver.

A.(nh4)(2co3)is the correct answer