How is a permanent cure for $ADA$ deficiency possible?

An example of gene therapy is

Euphenics is

Given below are two statements:
Statement $I$: $Adenosine \ deaminase$ is crucial for the immune system to function.
Statement $II$: $Adenosine \ deaminase$ deficiency can be cured only by bone marrow transplantation.
In the light of the above statements, choose the correct answer from the options given below.

$ELISA$ is used for the detection of viruses. The key reagent used is ......

What is the full form of $ADA$?