**Explanation:**

To determine which of the given options is not a heterogeneous fern, we need to understand the concept of heterogeneity in ferns.

**Heterogeneity in Ferns:**
Heterogeneity refers to the presence of two distinct types of sporangia (reproductive structures) on a fern plant. These types are called microsporangia and megasporangia.

- *Microsporangia:* These are small sporangia that produce microspores, which develop into male gametophytes (pollen grains).
- *Megasporangia:* These are larger sporangia that produce megaspores, which develop into female gametophytes.

Now, let's analyze each option to determine which one is not a heterogeneous fern:

**A. Selaginella:**
Selaginella is a genus of plants commonly known as spike mosses or resurrection plants. They are not true ferns, but rather lycophytes. Lycophytes do not exhibit heterogeneity. Therefore, Selaginella is not a heterogeneous fern.

**B. Salvinia:**
Salvinia is a genus of aquatic ferns. They have two types of sporangia, with the microsporangia and megasporangia borne on different parts of the plant. Salvinia exhibits heterogeneity.

**C. Azolla:**
Azolla is a genus of aquatic ferns commonly known as mosquito ferns. Similar to Salvinia, Azolla also exhibits heterogeneity with the presence of both microsporangia and megasporangia.

**D. Marsilea:**
Marsilea is a genus of aquatic ferns commonly known as water clovers. They have two types of sporangia, with the microsporangia and megasporangia borne on different parts of the plant. Marsilea exhibits heterogeneity.

**Conclusion:**
Based on the analysis, the option that is not a heterogeneous fern is **A. Selaginella** because it is not a true fern but a lycophyte. The other options, Salvinia, Azolla, and Marsilea, are all true ferns that exhibit heterogeneity.

Ncert ke according page no. 38 . selaginella and salvinia are heterosporous .. ..i think this question is incorrect