The trend of a variable over time is best illustrated by a:

Detailed Explanation:

To display a trend over time, a line graph is the best choice as it connects data points sequentially, showing the direction and pattern of change.

Option C: Line graph

Line graphs effectively display trends, helping viewers see how a variable changes over time.

Option A: Pie chart

Pie charts are used for showing proportions at a single point in time.

Option B: Pictogram

Pictograms display data using symbols, which are not suitable for trends.

Option D:Frequency distribution

Frequency distributions summarize data points but do not illustrate trends over time.