Which of the following is a Ghosh varna (घोष वर्ण - voiced consonant)?

Based on the vibration of vocal cords, consonants in the Devanagari script are classified into Aghosh (unvoiced) and Ghosh/Saghosh (voiced):
* Aghosh (अघोष): Includes the $1^{\text{st}}$ and $2^{\text{nd}}$ letters of every varga group (e.g., क, ख, च, छ, ट, ठ, त, थ, प, फ) along with श, ष, स.
* Ghosh/Saghosh (घोष): Includes the $3^{\text{rd}}$, $4^{\text{th}}$, and $5^{\text{th}}$ letters of every varga group (e.g., ग, घ, ङ, ज, झ, ञ, ड, ढ, ण, द, ध, न, ब, भ, म) along with nasals, semi-vowels (य, र, ल, व), and ह.

Let's check our options:

* 'च' $\to 1^{\text{st}}$ letter of Chavarga (Aghosh)
* 'ठ' $\to 2^{\text{nd}}$ letter of Tavarga (Aghosh)
* 'ख' $\to 2^{\text{nd}}$ letter of Kavarga (Aghosh)
* 'म' $\to 5^{\text{th}}$ letter of Pavarga (Ghosh)

Therefore, 'म' is a Ghosh varna.