Which Titan was Zeus's mother?

Rhea was Zeus’s mother.

Rhea was a Titan goddess, the daughter of Gaia and Uranus and the sister of Cronus. She and Cronus became consorts, and Zeus was their youngest child. Rhea also gave birth to Hestia, Demeter, Hera, Hades, and Poseidon, making her the mother of the six original Olympian siblings.

Cronus feared a prophecy that a child would overthrow him, so he swallowed each of his offspring. When Zeus was born, Rhea hid him in a cave on Crete and gave Cronus a stone wrapped in cloth. In some accounts, the Curetes masked the infant’s cries by clashing weapons and shouting. Zeus grew to adulthood in safety, then forced Cronus to release the other children.

The rescued siblings joined Zeus in the Titanomachy and eventually replaced the Titans as the ruling generation. Rhea should not be confused with Hera, who was Zeus’s sister and later wife, or with Leto, the mother of Apollo and Artemis. In Roman religion, Rhea was associated with Ops and was also linked with Cybele.