Branched-chain alkanes have lower values of ΔcH⊖ than straight-chain alkanes of the same number of carbon atoms, and so can be seen to be somewhat more stable. https://en.wikipedia.org/wiki/Alkane#Branched_alkanes
The heats of formation are given in https://en.wikipedia.org/wiki/Standard_enthalpy_of_formation and go from -40.0 kcal/mol for a to -41.8 for b to -42.5 for c to -44.5 for d. Compound d has the largest heat of formation, therefore will have the smallest heat of combustion.
The explanation is complicated. Sometimes you have to wonder why teachers spend so much time on effects that are so minor. The data are interesting, in a trivial sort of way. Show the data; it's interesting; move on.
Highly branched alkanes may stretch the bond angle. This can cause steric hindrance, and can substantially increase the reactivity. "Substantially". That's interesting. In addition, the reactivity of highly branched alkanes can be used to increase the octane rating of gasoline, because the stability of highly branched free radicals slows down the combustion and avoids explosions in the cylinder (pinging). Substantial is the key word, whether it it is reactivity or stability. The difference between 40 and 44.5 is hardly substantial. I would call that difference about half of substantial.
Wikipedia goes on: "However, in general and perhaps surprisingly, when branching is not extensive, branched alkanes are actually more thermodynamically stable than their linear (or less branched) isomers. For example, the highly branched 2,2,3,3-tetramethylbutane is about 1.9 kcal/mol more stable than its linear isomer, n-octane. Due to the subtlety of this effect, the exact reasons for this rule have been vigorously debated in the chemical literature and is yet unsettled. Several explanations, including stabilization of branched alkanes by electron correlation, destabilization of linear alkanes by steric repulsion, stabilization by neutral hyperconjugation, and/or electrostatic effects have been advanced as possibilities. The controversy is related to the question of whether the traditional explanation of hyperconjugation is the primary factor governing the stability of alkyl radicals." Surprisingly subtle and unsettled. Not really a great teaching topic.
The question regularly pops up in secondary school exams. Why? The reasoning seems unsettled. How to answer the question? The teacher must have given the answer if the teacher is at all competent and meant to provide the reasoning; so regurgitate the correct answer without reserving a special place in your memory for it. If the teacher did not provide the answer, then you must do your own research, in advance of the test - first, to discover what kind of questions your teacher is likely to ask (can you search out previous year's tests?) and second, to research the possible answers.
If you do well at this, you may be asked to teach the next class.