"is tightened" and "since" don't work together in English. Should be "was tightened", though even then the "since" sounds kind of awkward.
Past tense. Correct English is " Their defence WAS tightened two days ago"
"Their defense has been tightened for two days now" should also be accepted
Their defense has been tightened since two days ago.
the answer is not in correct English. ... since two days ago -> this is not English and wrong
This is not English!
Their defence tightened
MOBILE APP ERROR: All words already in correct position.