Bt toxins exist as inactive protoxins in bacteria but when an insect ingest the inactive toxin it becomes active toxin in the presence of :

The correct option is A Alkaline pH of insect mid gut
Bacillus thuringiensis is biopesticide, commonly known as Bt, is a naturally occurring, gram-positive, spore-forming soil bacterium. Bt has been known to be reservoir of several insecticidal proteins, such as endotoxins, cytolytic proteins, vegetative insecticidal proteins, etc.

During sporulation, it synthesizes a cytoplasmic inclusion containing one or more proteins that are toxic to insect larvae. Upon completion of sporulation the parent bacterium lyses to release the spore and the inclusion. In these inclusions, the toxins exist as inactive protoxins. When the inclusions are ingested by insect larvae, the alkaline pH solubilizes the crystal. The protoxin is then converted in to an active toxin after processing by the host proteases present in the midgut.

Bacillus thuringiensis forms protein crystals during a particular phase of their growth. These crystals contain a toxic insecticidal protein. These protein exist as inactive protoxins but once an insect ingest the inactive toxin, it is converted into an active form of toxin due to alkaline pH of the gut which solubilize the crystals. The activated toxin binds to the surface of midgut epithelial cells and create pores that cause cell swelling and lysis and eventually cause death of insect.
So, the correct answer is "Alkaline pH of gut".