To solve this problem, we need to determine which vector can clone only a small fragment of DNA. Let's analyze each option:$\newline \bullet$Option 1: Bacterial artificial chromosome (BAC)$\newline$ - BACs are used to clone large fragments of DNA, typically up to 300 kb.$\newline$ - Therefore, BACs are not suitable for cloning only small fragments of DNA.$\newline \bullet$Option 2: Yeast artificial chromosome (YAC)$\newline$ - YACs can clone very large fragments of DNA, up to 1 Mb or more.$\newline$ - Thus, YACs are not used for cloning small DNA fragments.$\newline \bullet$Option 3: Plasmid$\newline$ - Plasmids are small, circular DNA molecules that can clone DNA fragments typically up to 10 kb.$\newline$ - They are ideal for cloning small fragments of DNA.$\newline \bullet$Option 4: Cosmid$\newline$ - Cosmids can clone larger DNA fragments than plasmids, up to 45 kb.$\newline$ - They are not limited to cloning only small DNA fragments.$\newline$Based on the analysis, the vector that can clone only a small fragment of DNA is a plasmid.$\newline$Therefore, the correct option is Option 3: Plasmid.