Physical Properties of C and Si:
- Physical properties refer to the characteristics of a substance that can be observed or measured without changing its chemical composition. These properties include appearance, density, melting point, boiling point, conductivity, and hardness.
- Carbon (C) and Silicon (Si) are both nonmetals and belong to the same group in the periodic table (Group 14). Therefore, they have similar outer electron configurations, which leads to some similarities in their physical properties.

Appearance:
- Both carbon and silicon are solid at room temperature.
- Carbon exists in various allotropes, including graphite (soft, black, and slippery) and diamond (hard, transparent, and lustrous).
- Silicon is a shiny, grayish-black solid.

Density:
- Carbon has a lower density compared to silicon.
- The density of carbon depends on its allotrope. Graphite has a density of about 2.2 g/cm^3, while diamond has a density of about 3.5 g/cm^3.
- The density of silicon is about 2.3 g/cm^3.

Melting and Boiling Points:
- Carbon has a relatively low melting point of 3550°C and a boiling point of 4827°C.
- Silicon has a higher melting point of 1414°C and a boiling point of 3265°C.
- The difference in melting and boiling points can be attributed to the strength and type of bonding present in carbon and silicon compounds.

Conductivity:
- Carbon is a poor conductor of electricity and heat.
- Silicon is a semiconductor, meaning it has intermediate electrical conductivity between a conductor and an insulator.
- The conductivity of silicon can be modified by introducing impurities, a process known as doping.

Chemical Properties of C and Si:
- Chemical properties refer to how a substance reacts or interacts with other substances to form new substances with different chemical compositions.
- Carbon and silicon have different chemical properties due to the differences in their atomic structures and the nature of their chemical bonds.

Reactivity:
- Carbon is highly reactive and can form a wide range of compounds with other elements, including organic compounds.
- Silicon is less reactive compared to carbon and forms fewer compounds.

Chemical Bonding:
- Carbon forms strong covalent bonds with other carbon atoms, resulting in the formation of stable carbon-based compounds.
- Silicon also forms covalent bonds but is less likely to form multiple bonds compared to carbon. Silicon compounds are commonly found in minerals and materials.

In conclusion, while carbon and silicon share some physical properties like appearance and solid state at room temperature, they have different physical properties such as density, melting and boiling points, and conductivity. Additionally, their chemical properties differ due to variations in reactivity and bonding patterns. Therefore, the correct answer is option 'C': Same physical but different chemical properties.