The answer is C

We need present perfect tense here as we can see the effects of the operation still now.
We need since to indicate the continuity of time .
Hope it helps

Because we are talking about a time in the past until now, we have to use have/has + past participle when we use since.

C is the only option

Can someone explain all the options? I did not have good enough reason to eliminate any options

With "since" we use the present perfect or present perfect continuous. So A and B out.

With "after" we would use simple past in D.

The phrase is "subsequent to = after, following" in E, not "subsequently to".
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A & B out because sentence needs present perfect tense as weight loss effect is till now.
D is out because we need since with present perfect tense
In E Subsequently is an adverb if it would have been subsequent still since is preferred in present perfect tense as since shows continuation of time.

IMO C

We require present perfect tense so we narrow down to C,D,and E. C is best

Anyone please explain why the usage of "after" is wrong.

Q) Alfred lost more than 180 pounds since having an operation truncating his stomach three years ago.

We need to either
1) add "has" to make it a present perfect tense as it started in the past and continuing/vaild at present.
Alfred *has* lost more than 180 pounds since having an operation truncating his stomach three years ago.

(This is present in the options, so go with it)

Or

2) remove "since" and add "after" to make the sentence past tense.
Alfred lost more than 180 pounds after having an operation truncating his stomach three years ago.

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