We know that,
1 m = 100 cm
1 km = 1000 m
1 cm = 10 mm
$\therefore$ In the 4 given options, 1 m = 100 cm is correct.

We know that spoon is a small object which can hold very little quantity of liquid. 

Here, millimetre (mm) and centimetre (cm) are the units of length.

A litre is used to measure a large quantity of liquid.

Therefore, milk in a spoon is approximately 5 mL.

String `A` is starting from 1 cm.
$\therefore$ Subtract 1 cm from 9 cm.
Lenght of string A
$=9cm-1cm=8cm$
String `B` is starting from 3 cm.
$\therefore$ Subtract 3 cm from 10 cm.
Length of string B
$=10cm-3cm=7cm$
Thus, the difference in length of  String `A` and String `B`
$=8cm-7cm=1cm=10mm$
$\therefore$ The length of string A is more than string B by 10 mm.

Kilogram (kg) and milligram (mg) are the units of mass.

Millimetre (mm) is the unit used to measure the length of very small objects like the length of a pin, seed etc. 

Kilometre (km) is the unit used to measure long distances between two places.

When we convert a larger unit (L) to a smaller unit (mL), we multiply by the conversion factor.

Hence, 1 L = $1\times 1000$ = 1000 mL

Initial position of the ant = 2 cm

Ant goes  3 cm forward = 2 cm + 3 cm = 5 cm

Ant goes 2 cm backward = 5 cm - 2 cm = 3 cm

Finally, the ant goes 5 cm forward = 3 cm + 5 cm = 8 cm
$\therefore$ The final position of the ant is 8 cm

     $1\text{L}=1000\text{mL}$
$\Rightarrow 3\text{L}=3\times 1000\text{mL}$
            $=3000\text{mL}$
Therefore, the quantity of juice Tina wants to buy equals 3000 mL.
Since the store sells only 250 mL cartons, the number of such cartons that Tina has to buy is given by $\frac{3000}{250}=12.$

$\text{Weight of mangoes + 100 g + 500 g}=\text{1 kg + 2 kg}$
$\Rightarrow \text{Required weight + 600 g}=\text{3 kg}$
$\Rightarrow \text{Required weight}=\text{3 kg - 600 g}$
To move ahead, we need to convert the units. Since we need the final answer in kilograms, let us convert 600 g to kilograms.
        $1\text{kg}=1000\text{g}$
$\Rightarrow \text{600 g}=\frac{600}{1000}\text{kg}$
                $=0.6\text{kg}$
$\therefore \text{Weight of mangoes}=\text{3 kg - 0.6 kg}$
                                        $=\text{2.4 kg}$

When we convert a larger unit (hectometre) to a smaller unit (metres) we multiply by the conversion factor.
1 hectometre = 100 m

Hence 8 hectometres = 8 $\times$ 100 = 800 m

We know that, 
1 Litre = 1000 mL
We already have 350 mL,
$\Rightarrow$ 1000 mL - 350 mL = 650 mL
$\therefore$ 1000 mL = 350 mL + 650 mL