Step 1: Work out the relationship between the first pair.
A nucleus is the small, essential structure that sits inside a cell and directs its activities. So the link between the two words is simply that of a part lying inside a larger whole, nucleus is contained within cell.
Step 2: Carry the same relationship forward to yolk.
Ask yourself where a yolk is found. The yolk is the yellow portion that sits inside an egg, surrounded by the white and the shell. This is exactly the same part-inside-whole pattern as nucleus and cell.
Step 3: Rule out the remaining choices.
A grain is a whole unit in itself, not something that a yolk sits inside. A cob holds grains of corn, so the direction of the relationship is reversed. A fruit does not contain anything called a yolk. A seed is what a fruit contains, again the wrong direction and the wrong object.
Step 4: Pick the answer.
Only egg completes the analogy correctly, since the yolk lies inside the egg the same way the nucleus lies inside the cell. The answer is option (d) Egg.