If a single-celled organism is unable to undergo cell division, which of its life functions is affected?

The correct answer would be reproduction.

Single-celled organisms usually reproduce asexually with the help of cell division.

Asexual reproduction is a type of reproduction which takes place without the fusion of gametes and hence required only one parent.

Various types of asexual reproduction are fission (binary or multiple), budding, fragmentation, vegetative propagation etc.

Unicellular organism usually divide by fission either binary or multiple.

In binary fission, a parent cell divide into two equal sized daughter cells such as observed in bacteria.

In multiple fission, a parent cell divides into multiple daughter cells as observed in multiple fission of sporozoans etc. During this, parental nucleus divides several times with the help of mitosis without cytokinesis which produces several nuclei within a parent cell. The cytokinesis then takes place which results in separation and production of multiple daughter cells.

a physical change is being made.