1. It was —————that gave u the alarm. ( I, me)

It was me that gave u the alarm. I filled it with ME, but it was wrong answer. Please clarify me why should we use I instead of ME.

2. Whom can I trust, if not —————? (he, him). Please explain why should we use him instead of he.

3. He is as good as ———(I, me). I put it as me but it was wrong answer. Please clarify my doubt why should we use I instead of me.

3 answers

Disagree with 1 and 3.

The main difference between "me" and "I" is very easy to remember, as it simply refers to the pronoun type. More exactly, "I" is always used as a subject, while "me" is used as an object. This small and simple difference actually dictates in which situations you should use which.
If you rephrase each one so you're reading the very basic thought, you should come up with these:

1. I gave you the alarm. (You wouldn't say, "Me gave you the alarm," right?)

2. I can trust him. (Also, the "m" on "whom" gives this one away.)

3. He is as good as I am. (When comparing, make sure you add both verbs and the choice should be clear.)

Copy and paste this for personal pronouns and study the three paragraphs carefully:
guidetogrammar.org/grammar/pronouns1.htm#personal
Thank you so much