1)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
b)equally significant in the twentieth century has been
- I think its more meaning based and sentence formation. Since the first sentence has "led" (past) it doesnt make sense to have a present perfect in the next phrase.

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

The present perfect is inappropriate because in US English we only use present perfect to describe actions that began in the past but are still going on or have an effect in the present. Since the invention of a computer is not an extended action, but rather a pretty much one-time thing, (an invention occurs once, even if that invention is developed, elaborated, or improved upon) it is inappropriate to say "has been the invention of the computer".

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