Final answer:
The likely nucleophile used for the synthesis of oligonucleotides is sodium hydroxide (NaOH), as it is capable of deprotonating alcohols and initiating nucleophilic attacks necessary for oligonucleotide chain elongation.
Step-by-step explanation:
The nucleophile likely used in the preparation of oligonucleotides in the purification procedure would be one that can effectively attack and form bonds with carbon atoms in the growing oligonucleotide chain. Given the options provided, sodium hydroxide (NaOH) is a strong nucleophile and base that is commonly used to deprotonate alcohols and initiate nucleophilic attacks in many organic reactions. It would make sense for NaOH to be the likely nucleophile used for the synthesis of oligonucleotides in a chemical laboratory setting.
In contrast, (CH₃)₃CCl (tert-butyl chloride) is not a nucleophile but rather an alkyl halide, HCN is a weak acid (its anion, CN⁻, could act as a nucleophile but is not included in the options), and NaN₃ (sodium azide) is a source of N₃⁻ which is used for specific azide-related reactions rather than general oligonucleotide synthesis.