Cellular slime molds live as individual amoeboid cells when food is plentiful. when food decreases, the cells may come together by the thousand in a form called a slug. this slug possesses cells called sentinel cells, which display a primitive immune function because they:
a) circulate through the slug, engulfing bacteria.
b) inserting spikes into bacteria, causing lysis of the bacteria.
c) will produce antibodies in the presence of a virus.
d) release signal molecules similar to cytokine

a) circulate through the slug, engulfing bacteria.

Explanation:

Amidst slug cells, Sentinel cells acts as an elementary and competent immune system. These phagocytes goes round the slug body to capture toxic substances and infesting bacteria. It's also known to show the ability to release extracellular DNA traps around the invading/infesting body just ss its eeen in phagocytes of vertebrates and invertebrates.

The option A. Sentinel cells circulate within the slug, engulfing invading bacteria is correct as it also detoxify poisons or toxins, and removing and destroying such from the slug. Its operating mechanism in the cells is regulated by a Toll/Interleukin-1 Receptor domain protein (TirA).