Which element is a liquid at 758 k and standard pressure?
(1) gold (3) platinum
(2) silver (4) thallium

option (4) thallium.

Explanation:

1) You have to look at the normal boling and mleting points of the elements.

2) To be liquid the normal melting point must be lower than 758 K and the normal boiling point must be greater than 758 K.

3) The look at the data in a handbook of properties or in internet, and build a table.

4) The given temperature, 758K, is 758 - 273.15 = 484.85 °C.

Element                 Normal melting         Normal boiling        State at 484.85 °C

                               point (°C)                   point (°C)

(1) gold                    1064                           2700                      solid

(3) platinum             1768                            3825                      solid

Then, the only element of the choices whose melting point is less than 758 K (484.85) and its boiling point is greater than that, at 1 atm is thallium. So, it is liquid.

in scientific notation, the value of the  speed of light in a vacuum

  will be written as 2.995 * 10^8. when this value is represented in the form of

  a * 10^b, then, a = 2.995 and b = 8.

the value of the speed of light in a vacuum given above can also be rounded up to approximately 300000000. in this case, the scientific notation will be written as 3 *10^8.