smallest known moneran lacking cell wall are spirochaetes mycoplasmas cyanobacteria archaebacteria

The correct option is 2.
Mycoplasma refers to a genus of bacteria that lack a cell wall. Without a cell wall, they are unaffected by many common antibiotics such as penicillin or other beta-lactam antibiotics that target cell wall synthesis. They can be parasitic or saprotrophic. Several species are pathogenic in humans, including M. pneumoniae, which is an important cause of atypical pneumonia and other respiratory disorders, and M. genitalium, which is believed to be involved in pelvic inflammatory diseases. Mycoplasma are the smallest living cells yet discovered, can survive without oxygen and are typically about 0.1 µm in diameter.