To solve this problem, we need to analyze the given statements about mitochondria and chloroplasts:$\newline \bullet$Statement (a): Mitochondria and chloroplasts are semi-autonomous organelles.$\newline$ - These organelles contain their own DNA and ribosomes, allowing them to synthesize some of their own proteins.$\newline$ - They can replicate independently within the cell, which supports their semi-autonomous nature.$\newline$ - Therefore, statement (a) is true.$\newline \bullet$Statement (b): Mitochondria and chloroplasts are formed by division of pre-existing organelles and they contain DNA but lack protein synthesizing machinery.$\newline$ - While it is true that these organelles are formed by the division of pre-existing organelles and contain DNA,$\newline$ - It is incorrect to say they lack protein synthesizing machinery, as they do have their own ribosomes for protein synthesis.$\newline$ - Therefore, statement (b) is false.$\newline$Now, let's determine the correct option based on the analysis:$\newline \bullet$Statement (a) is true, but statement (b) is false.$\newline$Therefore, the correct option is Option 3: (a) is true but (b) is false.