6CO2 + 6H2O → C8H12O6 + 602 A
One compound
B.
One element and one compound
C с
Two elements
D
One element

B. One element and one compound

Explanation:

O2 is a diatomic element: "are pure elements that form molecules consisting of two atoms bonded together"

*there are 7 types of diatomic elements

and...

C8H12O6 is a compound

Chemical Compound can be described as  "a chemical substance composed of many identical molecules composed of atoms from more than one element held together by chemical bonds"

a mercury barometer is a device that uses the physical properties of mercury in a tube to measure barometric pressure. mercury barometers are used for : -

1. measurement of atmospheric pressure  

2. determining the altitude of a place

3. weather forecasting

4. calibration and checking of aneroid barometers

5. application in fluid mechanics

6. measurement of pressure in aircrafts

7. surface weather analysis

8. preparation of barographs

9. usage in physical sciences like physics, astronomy & chemistry

10. preparation of aircraft altimeters

characteristics of mercury barometer : -

1.used to define and measure atmospheric pressure.

2.on the average at sea level the column of mercury rises to a height of about 760 mm.

3.this quantity is equal to 1 atmosphere.

4.it is also known as standard atmospheric pressure.

5.a barometer in which the weight of a column of mercury in a glass tube with a sealed top is balanced against that of the atmosphere pressing on an exposed cistern of mercury at the base of the mercury column, the height of the column varying with atmospheric pressure.

i think it is true since you can test the answer and can use observations to complete it.