Question

For a long time, scientists thought that the depths of the ocean, for the most parts, was devoid of life.

• A. the depth of the ocean, for the most parts, was devoid of life.
• B. the depths of the ocean, for the most parts, were devoid of life.
• C. the depth of the ocean, for the most part, was devoid of life
• D. the depths of the ocean, for the most part, were devoid of life

Answer 1

The correct option is D the depths of the ocean, for the most part, were devoid of life

While ‘depth’ is a measure of the extent downward, ‘depths’ refers to the deepest or most remote part. The correct word in the given context is ‘depths’, not ‘depth.’
‘Depths’ is plural. So, the verb to follow this is ‘were’, not ‘was’.
The correct idiomatic usage is ‘for the most part’.