Although quite powerful in his time, the 16th century Russian czar Iva

I disagree...

The link https://www.chompchomp.com/terms/linkingverb.htm says, 'seem' is always linking verb. In that case, we can never use 'to be' with 'seem' but we have enough examples for 'seem to be'.

My choice for 'seem' versus 'seem to be' is based on the context.
For e.g., "Lakshmi seems a good girl" and "Lakshmi seems to be a good girl" both are correct. But then how do we decide which one to use? Answer is context.

If we know Lakshmi has done good things, we say she is a good gril (she seems good).
But if we know that Lakshmi has done those good things with some hidden agenda, we say 'she seems to be good'.

In the first case, it is certain that she is good, whereas in the later it is not certain.

Few more examples:

"things far off seem to be small"
"this post seems small"

Now applying this logic to our question: we know Ivan the Terrible is a powerful. With that he should be a strongly visible character in the history. But for some reason, historians did not talk about him which means his absence in the history is certain but not perceived. Hence there is no need to use 'to be' with 'seem'.

So, the correct answer is D.

Hope this is clear.