Which adjective means appearing to be true but not necessarily so?

The adjective "ostensible" is derived from the Latin root "ostendere," which means "to show." It describes something that is presented or appears to be the case but may not actually be true upon closer examination. This term is often used to indicate that something is outwardly apparent or seemingly legitimate while suggesting that the reality may differ from that appearance; in other words, it signifies a façade or an outward show that can be misleading.

In contrast, the other terms do not carry the same connotation. "Maunder" refers to talking or moving aimlessly or in a disorganized manner, lacking the quality of appearing true. "Maculate" means to mark or stain, which does not relate to the concept of appearance versus reality. "Mellifluous" describes a pleasing or musical sound, lacking any implication of truthfulness or appearance. Thus, "ostensible" is the fitting choice that captures the idea of something that seems true but may not be so in reality.