To solve this problem, we need to analyze each statement regarding factors affecting the rate of photosynthesis:$\newline \bullet$Option 1: Increasing atmospheric CO2 concentration up to 0.5% can enhance CO2 fixation rate.$\newline$ - This statement is correct. Increasing CO2 concentration can enhance the rate of photosynthesis up to a certain point, as CO2 is a substrate for the Calvin cycle.$\newline \bullet$Option 2: C3 plants respond to higher temperatures with enhanced photosynthesis while C4 plants have much lower temperature optimum.$\newline$ - This statement is incorrect. C3 plants generally have a lower temperature optimum compared to C4 plants. C4 plants are adapted to higher temperatures and have a higher temperature optimum for photosynthesis.$\newline \bullet$