I did not find any good any explanation in this thread so I am posting my thoughts here
Although the invention of the printing press led to a significant reconceptualization of textual production in the fifteenth century, what has been equally significant in the twentieth century is the invention of the computer.
(A) what
has been equally significant in the twentieth century is
(B) equally significant in the twentieth century
has been(C) no less significant in the twentieth century
is(D) What was equally significant in the twentieth century
were(E) no less significant in the twentieth century was
A note to remember about present perfect tenses
1. Among other things, the key aspect of this tense is the existence of a situation that started in the past and continues in the present(or its effect does). Any activity that is instantaneous cannot take a present prefect in its core form unless that activity has some effect, which can continue for longer(and is continuing).
In this question, the activity under discussion is "the invention of the computer". Now, as I mentioned before, invention is not a scattered activity.
You say the light bulb was invented in 1879.
You don't say - The light bulb has been invented in 1879. because it did not take a scattered amount of time across 1879 to invent a light bulb.
Similarly, the" invention of the computer" cannot take present perfect forms. That leaves us with C, D, and E. Between them, I think C and D are easy eliminations because of present tense and plural usage respectively. You have a winner in E. Sweet, easy, and harmless selection.
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