We had a . . . . . . . . of warm weather in March.

The correct answer is D. spell.

A spell is a period of time during which something happens or exists. In this case, the speaker is referring to a period of time during which the weather was warm.

The other options are not as accurate. A period is a length of time, but it does not necessarily have any specific meaning. A phase is a stage in a process, but it does not necessarily refer to a period of time. Time is a general term for a period of existence, but it does not necessarily have any specific meaning.

Therefore, the correct answer is D. spell.