When a chemical reaction and its reverse are occurring at the same time and at the same rate, the reaction has achieved
a. displacement.
c. imbalance.
b. equilibrium.
d. decomposition.

b. equilibrium.

Explanation:

When a chemical reaction and its reverse occur at the same time and at the same speed, the reaction reaches equilibrium. This is because both reactions end up nullifying their "influence" generating dynamic equilibrium, where all forms acting on the reaction are nullified. Thus there is no reaction sticking out in the system, so we can state that the reaction is in equilibrium.

you need to take a look at the periodic system in order to sort these elements into pairs that would most likely exhibit similar chemical properties. first, you need to find elements that are found in the same group (groups are vertical in the periodic system), and then to see whether they have the same valence number. if they meet these criteria, they will have similar properties.

the first pair: ne (neon) and kr (krypton), are noble gases found in group 8

the second pair: li (lithium) and k (potassium), found in group 1

the third pair: s (sulfur) and te (tellurium), found in group 6

the answer is option a.