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The primary cognitive deficit exhibited by patient H.M. following a bilateral medial temporal lobectomy was the inability to form new memories, also known as anterograde amnesia. This condition arose because the medial temporal lobes, which include structures like the hippocampus, are critical for the formation and consolidation of new declarative memories. Although H.M. retained his long-term memories from before the surgery, he could not create new explicit memories post-surgery.

The other choices refer to conditions that were not characteristic of H.M.'s case. Expressive aphasia pertains to difficulties in producing speech, which was not H.M.'s primary issue. Extensive loss of remote long-term memories would denote a failure in retrieving older memories; however, H.M. was able to recall memories from his past before the surgery. Severe receptive aphasia involves difficulties in comprehension, which also did not apply to H.M. His main challenge was specifically related to the formation of new memories after the surgery.