एकता में बल है।
Step 1: Recognize it's a proverb/general truth.
Proverbial statements in English are often expressed with a complete clause containing a verb.
Step 2: Choose natural proverb form.
The idiomatic rendering is There is strength in unity (also commonly "Unity is strength").
Step 3: Check grammar of options.
(A) Fragment—no verb.
(C) Grammatically okay but less idiomatic; misses the proverbial form.
(D) Adds modal "should," changing the meaning from a truth to advice.
Step 4: Conclude.
Hence, (B) best conveys the intended proverb with correct structure.
मैं एक पत्र लिखने जा रहा हूँ।