Which data do you need to calculate the wing load?
The wing load is defined as the weight on the wing per square metre. Therefore you need the total take-off weight and the flat surface of the wing to calculate it. For example, a 25m2 glider flown by a pilot with a TOW of 100kg has a wing load of 4kg/m2.

A pre-flight check should be executed...
Before you take off, you should check at least these things: pilot, lines, canopy, wind, airspace.

What part is indicated by C (the orange line)?
A diagonal rib connects the upper left corner of a cell with the lower right, or sometimes runs across two or more cells.
This allows the use of fewer lines, reducing the weight and drag of the paraglider.

A paraglider glides down through the air. But at what speed?
These are all speeds relative to the air!

Despite a good flare, you have a lot of speed just before touching down, because you landed with the wind in your back. What do you do?
A is the safest option if you're not familiar with a PLF (parachute landing fall) which needs practise. Your legs are much stronger than your back or arms.
Although on a flat grassy surface, you could get away with answer B, it can be very dangerous when stones, holes or other objects are hidden in the grass.
Using the protection in your harness as a landing option is not safe - would you park your car against a wall at full speed because the airbags will catch the blow?

Which pre-launch check is the most important?
Even though all newer harnesses have a safety feature to prevent you from falling out of the harness if you forget your leg straps, this is very uncomfortable and on an older harness it may be a fatal mistake. Checking lines and canopy is of course very important as well.
See the section on Launching in the Basic techniques chapter.

When is the best time to put your legs down during the landing approach?
D is the safest option. The others are either uncomfortable and unwanted because you can't use weightshift anymore, or unsafe if you are too late and land on your bum risking a back injury.

For a paraglider in the mountains, what wind speeds are considered safe to fly?
Wind results in turbulence in the mountains. While soaring on a ridge or at low dunes or flying in flatlands may be safe up to 4 Bft or a bit higher using a smaller mini-wing, in the mountains a wind speed up to 3 Bft (a maximum of 20 km/h) is the maximum for most pilots to fly safely. Strong winds are considered 25 km/h or over.

Why are the cell openings slightly pointed down?
The cell openings are positioned where the air flow hits the profile of the airfoil.

What is the weight of the fabric used to make paraglider canopies? (g/m2)?
Very light fabric strong enough for paragliders can be as light as 25g/m2. Very strong and heavy fabric weighs 40g/m2 or more.
At around 35g/m2, you can calculate that the weight of the fabric of just the top and bottom sail of a 25m2 canopy is a bit less than two kilos.
Your result is shown above.