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.

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.

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.

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

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

What happens when you fly too slowly?
Below the minimum speed or stall speed, the wing stops flying and will descend straight down like a parachute. This is a very unstable situation and often leads to a spin or stall.
See the chapters on Aerodynamics (stalls) and Advanced flying.

What happens when you brake a paraglider so much that the air speed falls below the minimum speed of about 25km/h?
The photo shows what happens when you pull so much brake that the air speed of the paraglider falls below its minimum speed. The glider stalls. Therefore we sometimes call the minimum speed of a paraglider the stall speed.
Don't do this while flying! Unless you are on an SIV- or acrocourse.
Older gliders can also enter a deep stall when you brake too much - see the chapter on Aerodynamics.

Which lines carry the most weight?
Most of the weight is carried by the lines attached to the front of the canopy.

What are the minimum requirements for a safe take-off?
A straight headwind is not necessary, you can take off in a slight crosswind. The runway should be clear of cables and other obstacles, but a 500m radius is not necessary. It is however essential to be able to abort your launch, for example when you notice a knot in the lines.
Standing on launch you see a rain shower approaching. Do you take off?
D is the only safe answer. You can never be sure that you have landed before the rain arrives, so C is risky. B is only partly true for light rains, but again you're taking a risk. A is not true, although flying in cloud is illegal in many countries.
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