1. Understand what a spring balance reads

A spring balance measures the tension in it.

2. For the lower spring balance

A block of mass $M$ hangs from the lower balance. In equilibrium, the tension in the lower balance must balance the weight of the block.

$T = Mg$

Since spring balances are calibrated in kg-wt (or equivalently show mass corresponding to weight), its reading is:

$M\text{ kg}$

3. For the upper spring balance

The upper balance supports the entire load below it, which is the lower balance plus the block. Since the balances are said to be light, their own weights are negligible.

So the upper balance also carries the same tension:

$T = Mg$

Hence its reading is also:

$M\text{ kg}$

4. Key idea

In a series arrangement of light balances, the same tension is transmitted throughout when the system is in equilibrium.

Thus:

• lower balance reads $M\text{ kg}$
• upper balance also reads $M\text{ kg}$

5. Check options

• A: Both the scales read $M\text{ kg}$ each ✅
• B: Lower reads $M\text{ kg}$ and upper zero ❌
• C: Sum of readings $= M\text{ kg}$ ❌
• D: Both read $M/2\text{ kg}$ ❌

Therefore, the correct option is A.