Step 1: Understand the request.
We must pick the correct Tatsam (pure Sanskrit) form of the everyday Hindi word ghoda.
Step 2: Recall Tatsam versus Tadbhav.
A Tatsam word is borrowed straight from Sanskrit, while a Tadbhav word is a softened, evolved form; ghoda is Tadbhav.
Step 3: Trace the evolution.
The Sanskrit source ghotak passed through Prakrit and softened into the spoken ghoda, with consonant and vowel simplification along the way.
Step 4: Weigh the options.
Ashva and baaji are Sanskrit synonyms for horse but are not the direct root of ghoda, and saindhav means a horse of the Sindh region.
Step 5: Identify the true root.
Only ghotak is the direct Sanskrit ancestor from which ghoda descended.
Step 6: Conclude.
Hence the correct Tatsam form is ghotak, option B.
\[ \boxed{\text{B. } \text{ghotak}} \]