Assertion (A) : Crystal protein Bt toxin does not kill the Bacillus but kills the insects. Reason (R) : Bt toxin exists in inactive form in Bacillus but when ingested by the insect, the acidic pH of its gut changes it into an active form.

The correct option is: Assertion (A) is true, but Reason (R) is false.

Step-by-step Explanation:

1. Analysis of Assertion (A):
The bacterium Bacillus thuringiensis (Bt) produces specific protein crystals during a particular phase of their growth. These crystals contain a toxic insecticidal protein called Bt toxin. Inside the bacterium, this toxin exists as an inactive precursor (known as a protoxin). Because it is inactive, it does not harm or kill the host Bacillus bacterium. However, once a susceptible insect ingests this protoxin, it is converted into an active toxin that kills the insect. Thus, Assertion (A) is true.

2. Analysis of Reason (R):
The inactive protoxin requires a specific environment to be converted into its active toxic form. When an insect ingests the inactive protein crystals, they enter the insect's gut. The midgut of the insect has an alkaline pH, which solubilizes the crystals. Midgut proteases then cleave the solubilized protoxin, converting it into an active, functional toxin. The active toxin binds to the surface of midgut epithelial cells, creating pores that cause cell swelling and lysis, eventually leading to the death of the insect.
Since the Reason (R) statement incorrectly claims that the acidic pH of the insect's gut activates the toxin, the Reason (R) is false.

Conclusion:
Assertion (A) is true because Bt toxin does not harm the bacteria but kills the insect, and Reason (R) is false because it is the alkaline pH (not acidic pH) of the insect gut that activates the Bt toxin.