To solve this problem, we need to analyze each statement given in the options:$\newline \bullet$Option 1: Horsetails are gymnosperms.$\newline$ - Horsetails belong to the genus Equisetum, which is a member of the group of plants known as pteridophytes (ferns and their relatives).$\newline$ - Gymnosperms, on the other hand, are seed-producing plants like conifers.$\newline$ - Therefore, this statement is incorrect.$\newline \bullet$Option 2: Selaginella is heterosporous, while Salvinia is homosporous.$\newline$ - Selaginella is indeed heterosporous, meaning it produces two types of spores: microspores and megaspores.$\newline$ - Salvinia, however, is also heterosporous, not homosporous.$\newline$ - Therefore, this statement is incorrect.$\newline \bullet$Option 3: Ovules are not enclosed by ovary wall in gymnosperms.$\newline$ - In gymnosperms, the ovules are exposed and not enclosed within an ovary. This is a defining characteristic of gymnosperms.$\newline$ - Therefore, this statement is correct.$\newline \bullet$Option 4: Stems are usually unbranched in both Cycas and Cedrus.$\newline$ - Cycas typically has unbranched stems, but Cedrus (a genus of conifers) usually has branched stems.$\newline$ - Therefore, this statement is incorrect.$\newline$Based on the analysis, the correct statement is Option 3: Ovules are not enclosed by ovary wall in gymnosperms.