Which of the following is NOT a type of radiation?

Delta radiation is not recognized as a specific type of radiation in the same way that alpha, beta, and neutron radiation are.

Alpha radiation consists of helium nuclei (two protons and two neutrons) and is emitted during certain types of radioactive decay. Beta radiation involves the emission of electrons or positrons, which are particles that can carry negative or positive charge, respectively. Neutron radiation consists of free neutrons being released during nuclear reactions or radioactive decay processes.

In contrast, "delta radiation" does not correspond to a standard classification in the context of radioactive decay or radiation types in nuclear physics. While the term might be used informally or in different contexts, it does not refer to a defined type of radiation like the others mentioned. This distinction clarifies why delta is the correct answer to the question regarding which term is not a formally recognized type of radiation.