limestone wrote:In 1896, Henri Bequerel found that Uranium salts emitted penetrating radiations similar to those which Roentgen produced only a year earlier with a ga discharge tube.

B. like those which Roentgen
C. similar to those that Roentgen had
D. similar to them that Roentgen
E. similar to those Roentgen

IMO: E. How to approach this problem?
[spoiler]OA: C[/spoiler]

I would have gone with C, if it didn't have 'had' in the end. The usage of 'had' is redundant since the sentence already says "produced only a year earlier ". This already tells you the timing of the events, and usage of 'had' not necessarily wrong, but seems redundant.
But due to lack of a better option, I'd go for C.
E is missing 'that' before Roentgen.

There is a sequence of past events.When ever 2 past events occur,we need to use Past Perfect for earlier past event and simple past for later past event.

Here "Roentgen had produced only a year earlier with a gas discharge tube" occurs earlier in past.So "had" is used.

Also Option E is missing "that".

Option C seems better.

Here,
since here we are talking about two past events. to show the relative time between two past events we use past perfect hence . the option ......... had ....... is correct

time ------>
----------------|------------------------|------------------------------------|-------------------------------------------|
.................past perfect..............past.......................................present..........................................future

The 2 best options are A and C.
I agree with beatthegmatinsept that "had" in C is redundant. But is not harmful and is not hampering the meaning. I would choose A over C because of - that vs which. Here which is used to describe a essential modifier.Normally in such cases that is better option.More on this in:
https://www.beatthegmat.com/that-vs-which-t12537.html

I am confusing between

similar to those and
similar to them

can somebody explain the above usage, which is correct at which situation.

According to me in a compound sentence
Difference between they and those is that
They is used when the subject of the previous subject is same as the next one.
Those is used when the subject of the previous subject is different from the next one.
In our problem,we are comparing 2 sets of particles,which are different.So THOSE is better.

Thanks RyabDark.
good concept, I was missing it.