A researcher is studying phases of the cell cycle in a population of cells during which there is an increase in the DNA content. This stage is most likely:

Interphase

Explanation:

The cell cycle is the series of events that occurs in a cell leading to its division. It is characterized by two main phases viz: Interphase and the Mitotic (M) phase.

The Interphase also called the resting phase is the phase that occurs between two successive divisions of the cell. It is that time where the cell prepares, grows and duplicates its genetic material (DNA). The duplication or replication of DNA occurs specifically in the S-phase or Synthesis phase of the interphase, in which the molecules of DNA in the chromosomes doubles, hence, increasing its content.

It is pertinent that DNA doubles in the cell prior to division (mitosis) because each resulting daughter cell needs to contain an equal and correct amount of genetic material as the parent cell.

Therefore, according to the question, the cell will likely be at the Interphase stage of the cell cycle since Its genetic material has increased.