Art historians

{A} - INCORRECT; "if" usage is used to "if.. then.." construct
{B} - INCORRECT; "if" usage is used to "if.. then.." construct
{C} - CORRECT
{D} - INCORRECT; "was" doesn't show the intended meaning; "as was done" seems to modify "altered" & "sketch" and should be rather "a practice"
{E} - INCORRECT; meaning issue - "a practice" is employed and not "done"

Hello!

The first split here is if vs. whether.
Though both are technically correct, the GMAT prefers to use "whether" in these situations, because "if" has additional meaning: to indicate condition.

Tell me if Tom calls.

In order not to create a confusion, the GMAT will always use "whether" in correct answers --> A and B are out.

The second split here is tense: whether it has been/had been/was altered since completion.

Past Perfect indicates that one action was completed before some time or before another action in the past. It doesn't fit here.
We have:
Art historians are using - Present Continuous
Leonardo da Vinci first sketched the figure in black - Past Simple
If we used Past Perfect, it would mean that the alteration happened before the picture was actually created --> B and E are out.

Now we need to choose between Present Perfect and Past Simple.

Past Simple indicates simply that something happened in the past. Usually (though not necessarily) a specific time of action is mentioned.
Leonardo da Vinci first sketched the figure in black. - He did it during his life, which is considered a specific time.

Present Perfect indicates that something has happened in the past and has impact on the present.
I have lost my keys. - It means that right now I don't have them.
There are some words that may indicate Present Perfect usage: already, yet, for, since.
The prices have risen since last summer.

The indicative word "since" and the intended meaning tell us that we should use Present Perfect here --> D is out.

Besides, the phrase "practice done" is incorrect in E.
This is an issue of word choice: to do practice or to employ practice. The latter is correct in the context given --> E is out again.