The mandatory combination of ions responsible for assembly of microtubules are Mg2+ and Ca2+

The intracellular structures involved in various movements in eukaryotic cells are called microtubules. These are filamentous in nature and involved in intracellular transport and cell division. Magnesium and calcium play an important role in microtubule assembly mainly formed from the protein gamma-tubulin. It is also influenced by the rate of GTP hydrolysis. Magnesium ions favour assembly whereas calcium ions inhibit assembly. Thus the correct answer is option B.