To solve this problem, we need to identify which plant has leaves modified into spines.$\newline$Let's analyze each option:$\newline \bullet$Opuntia:$\newline$ - Opuntia is a genus of cacti, commonly known as prickly pear.$\newline$ - In Opuntia, the leaves are modified into spines.$\newline$ - This modification helps in reducing water loss and provides protection from herbivores.$\newline \bullet$Pea:$\newline$ - In pea plants, the leaves are not modified into spines.$\newline$ - Instead, they have tendrils for climbing.$\newline \bullet$Onion:$\newline$ - Onion plants have bulbous leaves that store food.$\newline$ - They do not have leaves modified into spines.$\newline \bullet$Silk cotton:$\newline$ - Silk cotton trees have large leaves and do not modify them into spines.$\newline$Based on the analysis, the correct answer is Opuntia, where leaves are modified into spines.$\newline$Therefore, the correct option is Option 1.