To determine the rate at which [tex]\( A \)[/tex] forms in the given reaction [tex]\( A(aq) \rightleftharpoons B(aq) \)[/tex], we need to understand the concept of chemical equilibrium. At equilibrium, the forward and reverse reactions occur at the same rate. This means that the rate of formation of [tex]\( B \)[/tex] from [tex]\( A \)[/tex] is equal to the rate of formation of [tex]\( A \)[/tex] from [tex]\( B \)[/tex].
Given that [tex]\( B \)[/tex] forms at a rate of [tex]\( 0.5 \)[/tex] M/s, we can conclude that [tex]\( A \)[/tex] also forms at the same rate because the system is in equilibrium, and the rates of the forward and reverse reactions must be equal.
Thus, the rate at which [tex]\( A \)[/tex] forms is [tex]\( 0.5 \)[/tex] M/s.
So, the correct answer is:
[tex]\[ \boxed{0.5 \, M/s} \][/tex]
