Final answer:
Proficient word reading involves primarily phonological processing, which is recognizing spoken language sounds and their written representations, though orthographic mapping also plays a crucial role in recognizing words instantaneously.
Step-by-step explanation:
Proficient word reading requires using the mental process called phonological processing. This involves recognizing the sounds of spoken language and understanding how these sounds are represented by letters and spellings. Given the options provided, phonological working memory is also vital as it allows for the short-term retention of phonological information, but for proficient word reading, specifically, orthographic mapping is the process by which a skilled reader can instantly recognize whole words by their letters and sounds without conscious effort. However, the direct answer to this question is phonological processing as it is fundamentally tied to the ability to decode words when reading.