To 'tantalize' someone means to:

The term "tantalize" specifically refers to the act of teasing or tormenting someone by presenting something desirable yet keeping it out of reach. This word is derived from the myth of Tantalus, who was punished by the gods by being made to stand in water that receded when he tried to drink and beneath fruit that was eternally just out of reach. Thus, when someone is tantalized, they experience a mix of desire and frustration, as they see something they want but cannot attain it.

In contrast, other options imply completely different actions that do not align with the meaning of tantalize. Assisting freely suggests a supportive and helpful action, ignoring completely suggests a lack of attention or regard, and fulfilling desires instantly denotes an immediate gratification, all of which stray from the teasing and tormenting nature encapsulated in the correct definition.