Final answer:
The order of its size from largest to smallest is phospholipase C (1), phosphorylated tyrosine residue (2), diacylglycerol (3), and Ca^2+ ion (4).
Step-by-step explanation:
When arranging the given molecules in the order of its size, with "1" being the largest, we consider the complexity and typical molecular size of each entity in question. A phosphorylated tyrosine residue is a modified amino acid, which is smaller than a typical protein like phospholipase C but larger than the simpler molecules diacylglycerol and the Ca2+ ion. Phospholipase C is an enzyme that cleaves phospholipids into smaller molecules and is therefore larger than both diacylglycerol and a phosphorylated amino acid. Diacylglycerol (DAG) is a two fatty acid chain lipid molecule, larger than a single ion but smaller than a protein or phosphorylated residue. Lastly, the Ca2+ ion is the smallest among them. Based on this, the correct order from largest to smallest would be: (b) Phospholipase C, (a) Phosphorylated tyrosine residue, (c) Diacylglycerol, (d) Ca2+ ion.