Answer:
The correct option is 3. Capacity of smallest tank is 15-gallons.
Explanation:
The graph shows the relationship between capacity of tank and combined length of fish it can hold.
The line passing through (0,1) and (1,2).
Slope of the line is

Slope intercept form of a line is

Where, m is slope and b is y-intercept.
The slope of the line is 1 and the y-intercept is 1, therefore the equation of line is
..... (1)
If we want four 2-inch platys, three 1-inch guppies and a 3-inch loach, then the combined length of fish is

Therefore the combined length of fish is 14.
Put x=14 in equation 1.
Therefore, the capacity of smallest tank is 15-gallons and option 3 is correct.