- It was I who salted the earth around your flower deck. -- (if you want to sound formal)
- It was me that/who salted the earth around your flower deck. -- (which sounds less formal)
Both are correct, but because it's a test/question in a grammar book, and grammar books usually prefer the formal alternative, I think it wants this as the answer: It was I who ...
Note that the part "salted the earth around your flower deck" flows better with the formal alternative. A typical informal cleft-sentence of that sentence would be something like: "It was me who was responsible". (But It was me that/who salted the earth around your flower deck" is by no means wrong or incorrect.)
From Practical English Usage by Michael Swan:
131 cleft sentences (2): it was my secretary who ...
2 It is I who ... ; It is me that ...
When an emphasised subject is a pronoun, there are two possibilities. Compare:
- It is I who am responsible. (formal)
It's me that's/who's responsible. (informal)
- It is you who are in the wrong. (formal)
It's you that's in the wrong. (informal)
To avoid being either too formal or too informal in this case, we could say, for example,
I'm the person / the one who's responsible.