Step 1: Clarify the problematic term."Decollate" signifies beheading, which is nonsensical here. The sentence requires a word suggesting investigation or examination.
Step 2: Assess the choices.
delve: Used with "into," it implies researching or thoroughly inquiring. "Delve into the matter" is appropriate.
deduce: To conclude via reasoning. You deduce a fact, not "deduce into" a matter.
deflect: To redirect something. You deflect criticism or a question, not a matter.
Step 3: Select the best option."Delve into" is the optimal phrasal verb to represent a deep investigation of the matter.