Select the correct option:

The speaker’s explanation was so packed with technical jargon that it veered into __________.

The correct option is opacity.

To choose the correct word for the blank, let's analyze the context of the sentence and the meanings of the given options:

1. Context of the sentence:
The sentence describes a speaker whose explanation had so much technical jargon that it became difficult or impossible to understand. The word needed in the blank is a noun following "veered into" that means "obscurity," "unclearness," or "unintelligibility."

2. Analyzing the options:
• opacity (noun): The quality of being difficult to understand; obscureness or lack of clarity. Fits both grammatically (noun) and contextually.
• opalescent (adjective): Exhibiting a play of colors like that of an opal. Grammatically incorrect and contextually irrelevant.
• poaceous (adjective): Relating to or belonging to the grass family (Poaceae). Irrelevant to the context.
• opaque (adjective): Not able to be seen through; hard to understand. While it has a related meaning, it is an adjective, whereas the sentence requires a noun after the preposition "into".

Therefore, the noun opacity is the correct choice to complete the sentence.