Cells are in dynamic equilibrium. The answer is:
Cells alter metabolic fluxes in response to perturbation to restore metabolic reactions to an steady state.
Some reactions are actually maintained far away from equilibrium state.
Enzyme-limited reactions are often far from equilibrium, while substrate-limited reactions are near equilibrium.
Thermodynamically, A and D would have gone more towards the right hand side but it doesn't. That way they are most likely enzyme-limited.
Allosteric regulation and covalent modification cannot last that long, it has to have something to do with gene expression. Genes are transcribed into RNA and then RNA are translated into protein. The translation process is rather rigid, therefore the things that change is transcription. That's why the answer is
Altered transcriptional regulation of metabolic enzymes.
The official answer is allosteric regulation. Think about the Phospho-Fructo-Kinase that catalyze the conversion of Fruto-6-Phosphate to Fruto-1,6-Bisphosphate, the enzyme is activated by allosteric regulation by various energy state factors.
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