There are three different position isomers possible when two hydrogen atoms in benzene are replaced by two similar or different monovalent atoms or groups. Let's explore the reasons behind this answer.

Explanation:
1. Understanding Benzene Structure:
Benzene is a cyclic compound with a hexagonal ring of carbon atoms, and each carbon atom is bonded to a hydrogen atom. The molecular formula of benzene is C6H6.

2. Replacing Hydrogen Atoms:
When two hydrogen atoms in benzene are replaced by two similar or different monovalent atoms or groups, we need to consider the different possible positions for substitution.

3. Isomerism:
Isomers are compounds with the same molecular formula but different structural arrangements. In this case, we are looking for position isomers, which means the substitution occurs at different positions in the benzene ring.

4. Number of Possible Isomers:
To determine the number of possible position isomers, we need to consider the different positions where the substitution can occur.

a) Substituting two hydrogen atoms at adjacent positions:
If the two hydrogen atoms are substituted at adjacent positions, we get the same compound regardless of whether the substituents are similar or different. This is because the benzene structure has rotational symmetry, and the resulting compound can be rotated to obtain the same structure. Therefore, this results in only one isomer.

b) Substituting two hydrogen atoms at non-adjacent positions:
If the two hydrogen atoms are substituted at non-adjacent positions, we can have different arrangements depending on the positions chosen. This results in two possible isomers.

c) Combining the results:
Considering both cases, we have one isomer when the substitution occurs at adjacent positions and two isomers when the substitution occurs at non-adjacent positions. Thus, the total number of different position isomers possible is three.

Therefore, the correct answer is option 'C' - 3.