Really good question and I agree that it's a pretty subtle difference.

Here's one crack at explaining the difference:

*where "to put" is parallel to "remain", the controlling phrase of both of them is "have been able to" - and that's a pretty logical parallel set of items (they've been able to do two things, put capital in to tech and remain viable)

*where "have been able to" and "have remained" would be parallel treats them like separate events since they're not controlled by the same "have been able to" phrase. And that's a weird meaning since the two things are pretty clearly related in cause/effect. To use the phrasing in E and to convey a logical meaning you'd need to link them...something like "have been able to put capital into new tech and have therefore remained viable."

*and another thought on it - in D, there's absolutely no doubt over which two verbs are parallel since there are only two verbs left in the sentence (have been able to X and Y, and X and Y are in the same form). In E, you're trying to make parallel the first and third of three straight verbs, so it jumbles the meaning: "Have BEEN able to (put) and REMAINED". That intervening verb is at the very least one that puts a hitch in the parallelism, and so in direct comparison with D which doesn't have that problem, E is wrong.

good day Brian

Brian quote: *where "have been able to" and "have remained" would be parallel treats them like separate events since they're not controlled by the same "have been able to" phrase. And that's a weird meaning since the two things are pretty clearly related in cause/effect. To use the phrasing in E and to convey a logical meaning you'd need to link them...something like "have been able to put capital into new tech and have therefore remained viable."

i have one doubt : is it that the "have" of the "have been" in the first part of parallelism cannot be taken as ellipses in the second part and hence should be repeated in the second part as u have done so in the rewritten structure as bolded

in other words : is it true for any present perfect structure that the have of the present perfect in the first part cannot be taken as ellipses for the second part. for example in the following sentence : i have been going to the gym more than my brother have

in this sentence i need to repeat "have" in the second part to make it correct and cannot take it as an ellipse of first part

thanks and regards

D for me
In contrast to large steel...., the next part should start with small mills for a better comparison..so A, B and C are out.
E is wrong because "To put" isnt parallel to "remained"
D wins