As per Mendeleev’s prediction, atomic mass of eka-aluminium was:

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1. Mendeleev predicted an element beneath aluminium and named it "eka-aluminium" and estimated several of its properties.

2. He predicted its atomic mass to be $68$ based on gaps in his periodic table and periodic trends.

3. The element was later discovered (gallium) with an atomic mass about $69.72$, confirming Mendeleev's prediction was very close.

Therefore the predicted atomic mass of eka-aluminium was $68$.

FINAL_ANSWER: d

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