Question. Context: A lunar eclipse occurs when the Moon passes directly behind the Earth into its umbra (shadow). This can occur only when the sun, Earth, and moon are aligned (in "syzygy") exactly, or very closely so, with the Earth in the middle. Hence, a lunar eclipse can occur only the night of a full moon. The type and length of an eclipse depend upon the Moon's location relative to its orbital nodes.

Question: phase of the moon during a lunar eclipse?
Answer. a full moon
What step-by-step reasoning justifies that answer?
Reasoning: The relevant information to answer the above question is: Hence, a lunar eclipse can occur only the night of a full moon.